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While jayden james nude words for "with" in most Romance languages seem to be direct descendents from Latin "cum" e. Could it be that this french due to the fact that the latin obscenity for the female genitalia "cunnus" whose reflexes seem to survive as taboo word in most Romance languages also came to be pronounced cum, so that people tried to avoid the putative preposition "con" due to its obscene homophone?
It's probably hard to tell with certainty why this particular change took place, but I'd also be happy to know whether avoidance of words with taboo homophones is a well-known linguistic phenomenon and attested or at least conjectured in other cases. More important is the homophony with the word con in the meaning of the Modern French comparative adverb comme see e.
Le roman du renart. Same spelling was used also for what is Modern French quant "how much". You cannot confuse such highly frequent words as "with", "as" and "how much". Moreover, con in its obscene meaning seems attested comparatively later than cum other three french, just in the XIII cent. A known example from the history of German is the death of a whole class of composita both verbal and nominal with after- after After became a word meaning "anus" in the 19th century. This tradition is limited not only to Japan, but is widespread in the whole East Asian region.