Curiosity killed the cat
Inquisitiveness can lead one into dangerous situations.
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.