Green’s Dictionary of Slang

a person, a fellow, a chap.

1756		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 .
1849		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.
1930		‘Leslie Charteris’ Enter the Saint 55: I shan’t take nearly as long as that, my cabbage.
2017		L. McInerney Blood Miracles : ‘Fukcing Jason Pender, the poor fucking cabbage’.