Why do people say that cats have nine lives?

Cats are very resilient, and can survive various hardships, much more so than the other common house pet - dogs.. I guess people thought the cat would be dead, but it kept surprising them.

On the downsize, it gave people the impression they can do anything to a cat, which is of course total nonsense. 

Maybe because they're reborn as my Siamese (she wishes) appeared to be the day she scooted out from under a door. Sinbad had gone into an unused room and had been forgotten and came out the hard way.  She's still alive & just as ornery as ever. The two other cats & my little dog almost had heart attacks.