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Know Your Phrase has the meaning and origins for many popular expressions!
Common Phrase Meanings and Origin  |  C-Letter Sayings
Note: Tracking the origins for the majority of popular idioms isn't easy, in fact, it's often impossible. Usually what you will find are plausible theories that try to explain a phrase's origin, but remember, they are just theories.

The quotes on this website come from older books, poems, newspapers, etc. They can be used as an indicator to determine how old some of these sayings are. Keep in mind, however, that if a saying is being used in an old newspaper, it's probably already well-known, so it's likely older than that. - Common Phrase Origins - Sayings and Their Meanings
There is an older form of this expression that goes: "Care killed the cat." The word 
'care,' in this case, seems to be defined as "worry" or "sorrow." This form of the 
expression goes back to at least the 16th century. It's used by a few playwrights 
during that time. For example, an English playwright named Ben Jonson is said to 
have used it in a play called Every Man in His Humour, 1598. Another playwright 
named William Shakespeare used the expression in the play Much Ado About Nothing, which is thought to have been written in 1599:

    "What, courage man! what though care killed a cat, thou hast mettle enough in thee to kill care."

The earliest that I could find of the modern phrase being used in writing is from a book by James Allan Mair called Proverbs and Family Mottoes, 1891, where it's simply listed as a proverb on a page:

    "Curiosity killed the cat."
* My boss warned me that curiousity killed the cat after I kept pestering him to tell me why he fired his last employee.
Origins for this phrase.
Meaning of the phrase curiosity killed the cat.
Used as a warning for anyone who is acting excessively curious, as their prying behavior may lead them to harm or even death.
C - a list of sayings.
Phrases and Idioms