I heard somebody say “Du Kaschper” recently. What does it mean?
I think it could be some sort of German slang.
The word is actually written Kasper and often also pronounced Kasper — only the western part of the Bavarian dialect continuum and the Alemannic dialect speakers would pronounce it Kaschper due to the standard sound change of /s/ turning to /ʃ/ when preceeding /t/ or /p/. This sound change has been realised for initial /s/ only in standard German but southern dialects have adopted this change in various other positions, too.
The word has been used for many funny figures in children’s books and plays, most prominently for the titular Kasper (northern), Kasperle (Swabian) or Kasperl (Bavarian) of the Kasperltheater — corresponding approximately to Mr. Punch in Punch and Judy shows. Another very well known figure in Germany is Otfried Preußler’s children’s book der Räuber Hotzenplotz, which features Kasperl and Seppel as the two heroic main characters; Kasperl being the more playful just-do-it type frequently causing comic relief while Seppel is more thoughtful and anxious.
Etymologically, it derived from one of the three wise men, who were turned into kings and given names sometime in the middle ages. One of these three was termed Caspar and turned into an African. According to the etymological dictionary of German:
Kasper ist eine Nebenform von Kaspar (mlat. Casparus), dem legendären Namen eines der Heiligen Drei Könige aus dem Morgenland, der in den mittelalterlichen Dreikönigsspielen als Mohr dargestellt wird und (etwa seit dem 15. Jh.) die Gestalt einer lustigen Person annimmt. Ende des 18. Jhs. tritt er in Wien als komische Bühnenfigur an die Stelle des Hanswurst.
From these figures, colloquially and somewhat regionally the term Kasper(l(e)) has been derived as a derogative way of saying ‘you clown’. Depending on the speaker’s intention it can be anything from joking-affectionate to an outright insult.