Green’s Dictionary of Slang

(UK juv.) a catapult.

1898		Boy’s Own Paper 12 Nov. 73: Has everyone got catties and shot with them?
1911		Boys’ Best 20 Oct. 42: He thinks it was done with a bullet from a catty.
1988		Viz Oct./Nov. 12: Lummy! I’ll knock those down with my catty!