Green’s Dictionary of Slang

old boy n.

1. [17C+] (also old cove) an old or older man; esp. as the old boy, one’s father; often as a term of familiar address.

2. [late 18C+] (US/Irish) constr. with the, the Devil.

3. [mid-19C] the stuffing, the ‘daylights’, e.g. I’ll knock the Old Boy out of him.

4. [mid-19C; 1920s+] (also old chap) the penis [boy, the n.1 (1)].