People seem to lose the use of their native wit when they consider the origins of words. That idiot's sorting office, the Internet, has a well-intentioned site (at io.com/gibbonsb/words.words.words.html) edited by Gibbons Burke that discusses nautical terms used by Patrick O'Brian, who, Mr Burke remarks, uses expressions 'in a way that allows the reader to make the connection between a familiar phrase in everyday language with its marine heritage'. But when I read Patrick O'Brian's books three or four years ago, I was struck by how often his etymologies are wrong.
A contributor to Mr Burke's site quotes a sentence in supposed explanation of the phrase 'the cat is out of the bag': 'Vowles drew the cat from its red baize bag, phlegmatically took up his stance, and as the ship reached the height of her roll he laid on the first stroke.' The contributor suggests that this accords with the phrase as 'signifying that one has crossed some bright line of misconduct'. But that is not what the phrase means at all.
And if it is suggested that the 'cat' in the phrase is the cat-of-nine-tails, one only has to consider that the usual wording is 'let the cat out of the bag'. You might get a cat-of-nine-tails out of a bag but you couldn't let it out: it wouldn't jump. To be fair, Mr Burke does himself quote Brewer's Dictionary of Phrase and Fable, which suggests that the phrase derives from the notion of buying a pig (in fact a cat) in a poke.
The Oxford English Dictionary defines the meaning of to let the cat out of the bag as 'to disclose the secret', and its earliest citation is from 1760, from the London Magazine, though I wouldn't be at all surprised if earlier examples turned up if someone looked for them. It may be not without interest that there is a French phrase vider le sac, meaning 'to tell the whole story, finish the discussion'.
Some phrases are more doubtful. Thus to the bitter end is ordinarily taken to mean 'to the last and direst extremity; to death itself'. But the OED admits that the history is uncertain. In 1626 Captain John ('Virginia') Smith's Seaman's Grammar explains that 'the Bitter's end is that part of the Cable doth stay within boord'. That hardly helps with the figurative usage. Admiral W.H. Smyth in his Sailor's Word-Book (1867) says, 'When a chain or rope is paid out to the bitter-end, no more remains to be let go.' But the Admiral recruited any word he could lay hands on to the seafarer's arsenal. The most the OED says is 'hence perhaps bitter end' in the figurative sense. 'Perhaps' is an honourable word.