Originally, the 'kitty' at card-games was a percentage taken out of the pot to pay for the rent of the room the game was happening in, for refreshments and any other expenses. Nowadays, of course, it simply means the money staked. The origin of the word in this sense is unknown...or, at any rate, The Oxford English Dictionary - the word 'bible' - doesn't know what it is! (Other meanings are wench, wren, kitten and even prison.)