I've tried to find the etymology for Worauf and couldn't find the reason why Wo is used here instead of Was, for example.
Is there a story behind this or a source that explains where this comes from?
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What is important to note is that in Middle High German ("Mittelhochdeutsch", MHD), the locative meaning of today's wo was just one of several meanings that the corresponding word had historically. According to Henning's Kleines Mittelhochdeutsches Wörterbuch, MHD had an interrogative pronoun wô, (also attested as wâ and wâr) that translates to Modern High German wo ("where"), but also to forms like woher ("from where"), wenn ("when"), and wie ("how"). So, for a speaker of Middle High German, wô was not restricted to asking about places, but it could also be used to ask to time and manner for example.
This also holds for combined interrogatives such as wô vür (English "for what"): as wô could refer not only to locations but also to causes. Other non-locative uses include wô mit ("with what" or "by what") or wô von ("from where", but also "why"). In MHD, these combinations were meaningfully analyzable: a speaker of MHD could approximate the meaning of wô mit by combining one meaning of wô with the meaining of mit.
Over time, the meaning of wo changed and became mostly restricted to its locative meaning ("where") while the other meanings the interrogative had in MHD are mostly lost. At the same time, the combined interrogatives (wô mit, wô von etc.) became lexicalized: they lost their status as transparent, analyzable combinations that they had in MHD and changed into single lexical entries. The spelling as single string without a space is an indication of this lexicalization. In these entries, wo- acts more like a prefix than an interrogative pronoun. It's function may perhaps be described as indicating that the complete word is an interrogative pronoun with several different types of referents that are similar to the type of referents that wô had in MHD.
From the point of view of a morphologist, then, the best analysis of contemporary German perhaps is to assume two morphological units. The first unit is the interrogative pronoun wo(1) which refers to locations. The second unit is the prefix-like wo-(2) that you find in wofür, wovon etc. The meaning of wo-(2) has not undergone the meaning restriction to locations that wo(1) has seen. But unlike wo(1), it's not a free form but requires a base word with which it can combine to form an interrogative pronoun.
I guess, the answer is that German is conceptualizing the referents of these adverbs as places in space.
My assumption is backed by the following observations. However, the answer of Schmuddi claims that wo has had a broader meaning in Middle High German and just narrowed down to the spatial meaning later, and their claim seems to be correct.
Wo? as a question particle (meaning "where") correlates with da (meaning "there", but also "since" in its causal meaning and "hence"). Now all these adverbs / question words with wo have a counterpart with da: wo + [durch | her | hin | mit | rauf | raus | rüber | rum] can be answered with da + [durch | her | hin | mit | rauf | raus | rüber | rum]. That makes it rather improbable, that wo has been chosen here by accident, i.e. without a conceptual idea behind it (for instance simply due to mere phonetical reasons).
Nearly all the prepositions being combined with wo originally have a spatial meaning, and get a different, abstract, figurative meaning only through some analogy:
wodurch literally translates to "through what", since durch means "through" in the spatial meaning, and gets its figurative meaning "whereby", "by what", "by which means" only by analogy.
woher literally translates to "from where".
wohin literally translates to "where to".
worauf literally translates to "on where".
woraus literally translates to "out of what" (out in its spatial meaning here) and gets its figurative meaning "what from", "of what" by analogy.
worüber literally translates to "over what" - note that English differentiates the topical meaning and the spatial meaning in the words about and over, while German has only über to express both these concepts.
worum literally means "around what", so originally it also has a spatial meaning, and gets its figurative meaning "about what" by analogy.
If I didn't oversee and adverb with wo-, only the pair womit / damit does not have a literal spatial meaning, so, as a corollar to this working hypothesis, I would postulate, it is younger and has been modeled in analogy to the other adverbs.
In some dialects, wo is used as some sort of general relative pronoun. As far as I know, this is the case at least in Swabian dialect. This is a rather weak hint, and might not be strongly coupled to the phenomenon of the wo--adverbs. However, it provides another example where rather abstract concepts have been gained by an analogy to rather concrete spatial concepts.
Other languages have this analogy as well, however maybe to a lesser extent. For instance English has this. For example, notice that your question, asking
where this comes from
is actually using a spatial conceptualization to model a genetic relationship. If you look closely (!) into English, you will find a lot more examples for the phenomenon, that abstract ideas are modeled by an analogy to spatial concepts. Lakoff and Johnson's classic Metaphors We Live By has a bunch of examples of spatial metaphors which are basically engraved in the English language.
Lets start by noticing the three words Worum, Worauf and Worüber are both adverbs and compound words.
"Wo" by itself is also an adverb, and the first part of the compound. It does not function as a prefix in this case.
Lets split the words into their composite parts (the translations are approximate):
"Wo + rüber" (over what) the second part "Rüber" is also an adverb by itself.
"Wo + rauf" (on what), the second part "Rauf" is also an adverb or interjection.
"Wo + mit" (with what),the second part "Mit" is also an adverb or proposition.
"Wo + rum" (about what), the second part "Rum" is also by itself an adverb, although it's mostly used as part of a compound word, like "Wa + rum" (why).
Now finally addressing the question:
(...) why Wo is used here instead of Was, for example.
The simple reason is probably because compounding the words with "Was" instead of "Wo" wouldn't sound right. Put simply "Was" translates to "What", whereas "Wo" translates to "Where". If we notice closely in these words "Wo" takes on the meaning of "Was" and functions in its place. This sort of "exception" is easily possible mainly because both are interrogative adverbs, thus "function words", belonging to a closed and limited set.
Now a good comparison can be made with "Warum".
the etymology for Worauf
I admit this is a mystery to me and I look forward to someone explaining it.