Green’s Dictionary of Slang

a general term of address.

1939		P. Di Donato Christ in Concrete 206: ‘All right, cockos,’ commanded Fausta.
1941		G. Kersh They Die with Their Boots Clean 74: Laugh, my cock-oh [...] you’ll never see what I saw!
1957		G. Kersh Fowlers End (2001) 64: You don’t see a thing, cocko, you see a mess of blobs.