[Baron Munchausen]

While it is true that in stricter times one could be subjected to legal penalties for sex outside of marriage, the acronym explanation of the word is not valid. The euphemism 'carnal knowledge' does even not exist in English until the Bible becomes commonly available in English in the 16th century because it was the translation of a Hebrew verb that meant 'to know' and also had the connotation of sexual intercourse that created that new connotation in English. William Tyndale was burned as a heretic for translating the Bible into English, btw. And then a few years later King James comissions an 'official' translation and his 'team of scholars' rips off reams of Tyndale's work... go figure.

Anyway, the word is considered a cognate of a German word 'ficken' and to have been influenced by a Latin word 'futuere'. Both of these words mean essentially the same thing -- 'to strike' -- which makes perfect sense for the action described. I think 'ficken' is actually used in German just as we use 'f-ck' too.