One hypothesis is that to fuck in English and the similar use of ficken in German, originated in several Germanic speaking places in Europe at about the same time during the middle ages, among students, who were all in minor religious orders, who knew Latin, and who otherwise spoke Germanic languages. The words are simply student slang deriving from the dog Latin amorem facere meaning to make love. Because to begin with the words were somewhat obscure and inoffensive, usage spread widely and the knowledge of their origin was soon lost. Subsequently, their sexual meaning turned them into rude words, but ficken rather less so than to fuck. Fuck degenerated into an insult, its use was frowned on, so it stayed largely out of printed sources until the 20th Century. 
 There is no specific evidence for this hypothesis apart from the usage of the word in the languages concerned over many centuries. However, it seems far more likely to be true than the various alternative suggestions, unless it can be clearly ruled out on linguistic grounds. Could this hypothesis be circulated more widely and hopefully attract critical comment?