Green’s Dictionary of Slang

cabbage n.3

[Fr. choux. lit. ‘cabbage’, used as a term of endearment]

a person, a fellow, a chap.

[UK]Johnson Connoisseur No. 138 6: Those who [...] call a man a cabbage, a crab, a queer cub, an odd fish, and an unaccountable muskin, should never come into company without an interpreter .
[UK]Sam Sly 17 Mar. 3/1: Leave the barber alone, he is not enraptured with you, far from it. Mend the holes in your stockings, and look less after the cabbage.
[UK]‘Leslie Charteris’ Enter the Saint 55: I shan’t take nearly as long as that, my cabbage.