My main concern was in understanding why being a homeless person should be offensive, or is there another hidden meaning?
I share the doubt. In my personal experience the connotation is not usually intended.
A girl who calls someone a blöder Penner simply uses an insult . There is no deeper meaning. It's in how you say it--Der Ton macht die Musik.
There may be an association to the trodden down, insufferable apperance of certain beggars, but chances are the term was already negative before it was applied to people sleeping in the streets.
The answers claiming otherwise are unsubstantiated; appreciable, but formally inadmissable.
These comments are likewise unsubstantiated:
Note that outdated youth slang term Penne for ‘school’ is usually not associated with Penner, neither when used as an insult nor for homeless people.
there is "Pennäler" which would indeed refer to a schoolchild. Unrelated to Penner
They are correct insofar the interjection is used without any further association! The only association is what can be learned from its broad usage, which hardly includes any scolding of vagabonds. Like F@~k this $#!t, slurs are rarely meant to be taken literal, without doubt.
Confer Pfeifer on pennen:
pennen Vb. ‘schlafen’, umgangssprachlich (19. Jh.) aus der Gaunersprache; vielleicht zu jidd. pannai ‘müßig’, hebr. penaj ‘(freie) Zeit’, eigentlich ‘Zeit zur Muße’, wenn nicht besser Ableitung von
Penne2 f. ‘Kneipe, schlechte Herberge, Schlafstelle’ (Anfang 19. Jh.), älter Benne (Mitte 18. Jh.); aus der Gaunersprache, vgl. Bonne ‘ein Haus, wo Spitzbuben ein und aus gehen’ (Ende 17. Jh.); auch stille Penne ‘Gefängnis, Zuchthaus’ (dieses nach zigeuner. štilepen ‘Gefängnis’?). Vielleicht zu […]
Dazu Pennbruder m. ‘Landstreicher’ (Ende 19. Jh.) und Penner m. ‘Landstreicher, Schlafmütze’ (Anfang 20. Jh.).
[„pennen“, in: Wolfgang Pfeifer et al., Etymologisches Wörterbuch des Deutschen (1993), digitalisierte und von Wolfgang Pfeifer überarbeitete Version im Digitalen Wörterbuch der deutschen Sprache, https://www.dwds.de/wb/pennen, abgerufen am 02.02.2020.]
So it's true that Penner in its current form is whitnessed only turn of the last century, but the underlying notion is older and essentially unknown.
The given comparisons are good guesses, but weak insofar Gaunersprache (e.g. Rotwelsh) is notoriously hard to pin down because it's chiefly oral and secretive by nature.
A derivation of the verb from the noun is not even unlikely, if used as adverb of place, ich geh Penne.
Pfeifer further concedes to seeking Penne elsewhere:
Penne1 f. ‘höhere Schule’, scherzhaft in der Schülersprache (heute veraltend). Aus mlat. pennale ‘Federbüchse’ (in der Lateinschule aufkommend), dem substantivierten Neutrum von mlat. pennalis ‘zur Schreibfeder gehörig’ (zu lat. penna ‘Feder, Flügel’, spätlat. ‘Schreibfeder’), wird Pennal n. ‘Federkasten, Federbüchse’ (15. Jh.) entlehnt. Daraus bildet die Studentensprache scherzhaft Pennal m. ‘Student der ersten Semester, der alle Vorlesungen nachschreibt, deshalb stets sein Schreibzeug bei sich führt’, spottend von älteren Studenten verwendet (Mitte 17. Jh.), später (in der Schülersprache) ‘Gymnasiast, angehender Student’ (19. Jh.) sowie (mit neutralem Genus) ‘Gymnasium’ (Mitte 19. Jh.). Die verkürzende Eindeutschung Penne f. ‘höhere Schule’ entsteht wohl unter Einfluß von
Penne2 ‘Herberge’ (s. d.). Ebenfalls aus Pennal m. ‘Gymnasiast’ geht, zunächst als Schimpfwort, Pennäler m. ‘Schüler’ (Mitte 19. Jh.) hervor; heute ebenfalls in seiner Gebrauchshäufigkeit zurückgehend.
[„Penne“, in: Wolfgang Pfeifer et al., Etymologisches Wörterbuch des Deutschen (1993), digitalisierte und von Wolfgang Pfeifer überarbeitete Version im Digitalen Wörterbuch der deutschen Sprache, https://www.dwds.de/wb/pennen, abgerufen am 02.02.2020.]
Now you surely noticed that the two given etymologies contradict each other. That does not necessarily mean that they are wrong. Rather, it's difficult to say how they could be both correct. If (and only if) we accept that there might be mutual influences should we consider there might be even more:
It's hear-say, but I think Penne used to mean the Karzer (penetentiary facility) in certain schools.
En pen (chicken coop) means enclosure since Old English at least. This has been compared to pin reconstructing *penno
Proto-Germanic *pennō, *pannijō (“pin, bolt, nail, tack”), from Proto-Indo-European *bend- (“pointed peg, nail, edge”). Akin to Old English pennian (“to close, lock, bolt”) (in compounds onpennian (“to open”)), Low German pennen (“to secure a door with a bolt”)
[https://en.m.wiktionary.org/wiki/pen bold emphasis mine]
Compare similarly Schalter (switch), Bankschalter (Kasse), Schlüssel (key), Schloss (castle), etc.
The slur is likely not so old that PIE need concern us, but for completeness sake note that the existence of *b in PIE is controversial. The root is not indexed in the wiktionary and the claim not sourced, but Texas University's IE Lexicon is indexing *bend- (either often following Pokorny's IEW, which is partially outdated), though showing no descendents other than Germanic, e.g. OE pund "pound", and pintel "penis". The loss of *-d- is not obvious to me. More or less similar roots are indexed: *bak-, *bhendh, *bhedhH-, perhaps *penkwe-, *(s)pen(-d)-, *spend-, *pen-, *bhudh-, *bhew- (as indirectly implied by Pfeifer), *(s)pingo-, *h2epi- ... As the saying goes: Everything looks like a nail if all you have is a hammer (or just if you have a hammer). Compare Spunt, Spint; also, err, sphincter (Schließ-Muskel).
also cp Stift, Stippie, Stepke and Stiftung, Stift, by the way, further Stöpsel, Stopfen and Stoppel, En stubble, stub, further stumpf, dumpf, dumm ... Dom?
Ger Hahn im Korb is likely cognate to coop, possibly also related to ab in's Körbchen, but that's not enough to compare Schlaf to Schloss, or anything like that.
Spinner, spinnen: another slur of similar shape, without any obvious connotation. Lying, or creative interpretation, is associated e.g. through Seemansgarn (seeman's yarn, tell tale), further spinning wheel. Compare *bhendh- "to bind, bond" noted above. Having worked in a factory too long I found that it would easily derive from an occupational title, but only a Spinner would think so.
Eng. spinner may mean tramp, hooker; Incidently, hooker might relate to [hitch] hike (walk the line, Ger auf den Strich gehen). English however says spinster for the occupation in textile; cp Ger Gespenst, *(s)pen(-d)- noted above? Be a bit spontanious? Also cp. spine, spindle (viz peg, pin above).
puny, adj. "inferior ...", n. "A new pupil at a school etc; ...", from Middle French puisne, "contraction of puis (“later”) + né (“born”)", from Latin, from "post" according to wiktionary; However, cp *peH- "small".
A word being considered an insult because it refering to sleep? I wish Langschläfer, Schlafmütze, Tagträumer and Schläfer were considered offensive ;-) – dakab Jan 16 '16 at 22:04
@dakab: Depending on the context, at least Schlafmütze and Tagträumer can indeed be considered offensive. […] – O. R. Mapper Jan 16 '16 at 23:06
Although this is correct, sleepy heads themselves rarely are offensive.
The term generally implies bad manners, which of course readily allows association with all kinds of low-lifes, outlaws, people who cut the line and miss the light turning green.
Implied above: Penner is often used by women towards sexist men; Usage is paramount for interpretation. Notably it's similar to Penis, insofar dysphemistic slurs often involve genitalia. I thinkt that's not intentional nor subliminal either, but hypotheticly an early influence. Typical example: Jasna Fritzi Bauer towards Taktloss in Loyale Nutten. Also compare arroganter Pinsel.
It's beyond doubt that Schülersprache (see Pfeifer above) would enjoy a good double entendre. IMHO it's beyond reasonable doubt that penna was not one. Just how old it might be is hard to say, if pencil from a sense "tail" gives yet another parallel.
Further, I don't want to touch upon penalty, but the association to jail is there in Pfeifer's notes. I have to remark that penetentiary officers may naturally be the referrent of insults, as much as are teachers.
Speculation usually opens up Pandora's Box, so it should be no surprise that there's no conclusion. But perhaps that's precisely my point, that the application of the slur to poor temporary homeless people and further interpretation, which others already explained, must be deemed coincidental for the time being.