“Cat has nine lives” refers, as we all know, to the resilience and toughness of the domestic and feral cat. It indicates that they have more than one life such is their ability to survive.
The reason why nine was chosen is because in ancient times nine was considered a particularly lucky number. It was a trinity of trinities. The number nine, therefore, was considered lucky which reflected the luck of a cat to survive unforeseen life-threatening circumstances.