The popular story about carnival is the nicer one. It is probably wrong.
The nice version says the word comes from Medieval Latin carne vale, roughly "flesh, farewell." Etymonline mentions it only as a folk etymology. Vocabulary.com traces it to John Florio's 1611 Italian-English dictionary, which glossed carnevale as the time when flesh is bidden farewell. Lord Byron repeated it in Beppo in 1817. The article says there is no historical basis for it.
The better-supported theory points to carnem levare, "the taking away of meat." Vocabulary.com says the earliest evidence is 12th-century northern Italian dialect forms such as carnelevale and carnelevare. A syllable then dropped out, a process linguists call haplology, and the word settled as carnevale. Etymonline agrees that the Italian word likely comes from older forms meaning "to remove meat," from Latin caro, "flesh," and levare, "to lift or remove."
English took the word from French carnaval, which came from the Italian. Etymonline dates it to the 1540s, when it meant a time of merrymaking before Lent. By the 1590s it could mean feasting or revelry in general.
There is a third, more daring theory. The French historian Philippe Walter argues that the word is older than Christianity. In his view "the taking away of meat" was a Christian gloss on earlier pagan rituals tied to the Roman goddess Carna. Vocabulary.com calls this educated conjecture, and it remains a minority view.
The word also left the church calendar. Etymonline records that in American English, by 1926, a carnival could mean a circus or amusement fair. The slang carny, for a carnival worker, dates to 1931. The fair on the edge of town still has the old word, though nobody there is giving up meat.