The devil.
1802. The devil has been nick-named the old boy, perhaps, by some, as sounding more modish, familiar, or polite, and not bearing so hard upon him as his proper name…. His impudence in lying proves him to be an old boy.—The Balance, Oct. 4, p. 317/1. (Italics in the original.)
1833. They keep more honest men from heaven than the very old boy himself?—J. K. Paulding, ‘The Banks of the Ohio,’ iii. 65 (Lond.).
1858. Ah, yes. I see you forget me. I have the pleasure of being the Old Boy, at your service, otherwise called Sir Urian, familiarly known to my friends as Old Nick, Auld Hornie, Clootie, and such like. Others call me the Adversary, the Slanderer, the Father of Lies, but among my oldest acquaintances, the Hebrews, I am betitled the Prince of Flies and—.—Yale Lit. Mag., xxiii. 184 (April).
