(NSFW Language) Are the some of the oldest words in the English language really acronyms?

A co-worker of mine said he heard on “the Discovery Channel” (I guess it must be true!) that the word “shit” was really an acronym for “Ship High In Transit”. This reminded me of something I heard long ago about the work “fuck”, which supposedly stood for “For Unlawful Carnal Knowledge”. Now these are supposedly words from earliest Anglo-Saxon roots and earlier. I find it hard to believe that such old words really started out as abbreviations for such concepts. What do you think?