It's a metaphor – cats aren't actually immortal, but they have excellent reflexes and agility, which helps them avoid danger and escape from falls and other accidents.
Here's why the saying might have originated:
* Cats' reflexes: Cats are known for their quick reactions and ability to right themselves in mid-air, allowing them to land safely after falls.
* Their ability to survive: Cats are surprisingly resilient animals and can survive injuries that would be fatal to other creatures.
* Their nocturnal habits: Cats are active at night, and in the dark, they can easily slip away from danger, leading people to believe they have supernatural abilities.
While the saying is just a fun saying, it's important to remember that cats are still vulnerable to injury and illness. Always care for your feline friend and keep them safe!