Green’s Dictionary of Slang

caterpillar’s raincoat n.

(UK black) a condom.

1999		(con. 1979–80) A. Wheatle Brixton Rock (2004) 86: You don’t have to fret ’bout getting big belly, ’cos I always carry a caterpillar’s raincoat.
2001		(con. 1981) A. Wheatle East of Acre Lane 285: I ain’t got no caterpillar coats, man.