Everyone knows that, despite its supposed nine lives, curiosity killed the cat. Well, not quite. The 'killed the cat' proverb originated as 'care killed the cat'. By 'care' the coiner of the expression meant 'worry/sorrow' rather than our more usual contemporary 'look after/provide for' meaning.
That form of the expression is first recorded in the English playwright Ben Jonson's play Every Man in His Humour, 1598:
"Helter skelter, hang sorrow, care'll kill a Cat, up-tails all, and a Louse for the Hangman."
The play was one of the Tudor humours comedies, in which each major character is assigned a particular 'humour' or trait. The play is thought to have been performed in 1598 by The Lord Chamberlain's Men, a troupe of actors including William Shakespeare and William Kempe. Shakespeare was no slouch when it came to appropriating a memorable line and it crops up the following year in Much Ado About Nothing:
"What, courage man! what though care killed a cat, thou hast mettle enough in thee to kill care."
The proverbial expression 'curiosity killed the cat', which is usually used when attempting to stop someone asking unwanted questions, is much more recent. The earlier form was still in use in 1898, when it was defined in Brewer's Dictionary of Phrase and Fable:
"Care killed the Cat. It is said that a cat has nine lives, but care would wear them all out."
Curiosity hasn't received a good press over the centuries. Saint Augustine wrote in Confessions, AD 397, that, in the eons before creating heaven and earth, God "fashioned hell for the inquisitive". John Clarke, in Paroemiologia, 1639 suggested that "He that pryeth into every cloud may be struck with a thunderbolt". In Don Juan, Lord Byron called curiosity "that low vice". That bad opinion, and the fact that cats are notoriously inquisitive, lead to the source of their demise being changed from 'care' to 'curiosity'.
The earliest version that I have found of the precise current form of the proverb in print is from The Galveston Daily News, 1898:
It is said that once "curiosity killed a Thomas cat."
[Thomas cat is a jokey form of tom cat, i.e. a male cat.]
The frequent rejoinder to 'curiosity killed the cat' is 'satisfaction brought it back'. I've not been able to trace the source of this odd reply. The first citation of it that I've found in print is from an Iowan college magazine The Coe College Cosmos, in February 1933.
http://www.phrases.org.uk/meanings/curiosity-killed-the-cat.html
Curiosity killed the cat reminds us that being too curious can be dangerous. Example: "What do you think is down that dark street?" Reply: "I would rather not find out. Curiosity killed the cat."
Curiosity killed the cat recalls a story in which "the cat" was killed because he was too curious and followed "curiosity" too far. Example: "That reporter has been asking a lot of questions and the boss doesn't like it." Reply: "Curiosity killed the cat."
Cats are curious animals that like to investigate, but their curiosity can take them places where they might get hurt. Children especially, like cats, are curious and like to test to find out what is dangerous. Example: "My son stuck his finger into the electrical outlet and got a huge shock! He said he wanted to find out how it would feel." Reply: "It's a good thing he wasn't hurt! Curiosity killed the cat."
http://www.goenglish.com/CuriosityKilledTheCat.asp
‘What'd you ask 'em, for instance?’‥‘Curiosity killed a cat! Ask me no questions and I'll tell you no lies.’
[1921 E. O' neill Diff'rent ii. 252]
‘A curiosity death,’ said Tommy. ‘Curiosity killed the cat.’
[1973 A. Christie Postern of Fate i. iv.]
‘I'm curious, that's all.’ ‘Curiosity killed old tom.’
[1984 J. R. Riggs Last Laugh iii.]
‘Well, you've probably heard the saying, “curiosity killed the cat”. Well that's what I'm doing here.’ For one horrible minute, I thought that he was about to‥reveal that he was Ben Vol-au-Vent from Curiosity Killed the Cat, killing a cat.
[2002 Times 2 17 May 9]
Idioms: curiosity killed the cat
It's best to mind one's own business. For example, Don't ask about his divorce--curiosity killed the cat. This cautionary expression sounds like the moral of some fable or folktale, but any such origin for it has been lost. The first recorded use was in O. Henry's Schools and Schools (1909).
http://www.answers.com/topic/curiosity-killed-the-cat