Green’s Dictionary of Slang

1. a general term of address, usu. to a man [cock n.² ].

1899		C. Rook Hooligan Nights 39: You slip in there, cocker.
1952		C. Harris Death of a Barrow Boy 40: ‘’Allo, cocker,’ said Hutchens.
1962		A. Wesker Chips with Everything I vii: Cannibal has spoken, come on, me ole cocker, say more.
1978		A. Burgess 1985 (1980) 148: You wait here, cocker, till court in the morning.
1986		K. Royce Mosley Receipt 165: ‘Hello, cocker,’ Bulman said warmly as he shook Scott's hand.
1999		Indep. Rev. 22 July 8: Come on now Cocker. Clean yer railings.
2010		A.J. Rock Psychosomatic Crabs 58: In came Mr. Twigg. ‘Hello cocker,’ he said to Alex, ‘And Terry! how's that swimming going?’.

2. the penis [cock n.³ (1)].

1969		S. Defesse Henriette n.p.: ‘That kid upstairs hasn’t got a cocker like I’ve got to give you a good filling’.
1976		A. Cooper Abuse of Maryjane n.p.: Burrowing into his trousers to force them open and unsheathe his pulsating cocker, glistening with body juices.
1984		B. Randolph Best Balling Wife n.p.: [She] sucked tenderly on the bulbous cocker.