Green’s Dictionary of Slang

monkey’s cousin n.

[rhy. sl. = a dozen]

(bingo) the number twelve.

1981		P. Wright Cockney Dialect and Sl. 109: 12 = monkey’s cousin.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.