Green’s Dictionary of Slang

1. (also old gentleman in black) the Devil; also in fig. use (see cite 1891).

1680		Dryden Spanish Fryar V ii: ped.: Gomez, give way to the old gentleman in black. gom.: No! the t’other old gentleman in black shall take me if I do; I will speak first!
1700		T. Brown Amusements Serious and Comical in Works (1744) III 102: I know not who’ll take ’em [i.e. Presbyterians] for saints, but the old gentleman in black.
1727		De Foe Hist. App. (1729) 364: The devil is not so black as he is painted, but that you may form such images of the old gentleman [F&H].
1838		‘Lay of St. Nicholas’ in Bentley’s Misc. Apr. 498: And how, to the day of their death, the ‘Old Gentleman’ / Never attempted to kidnap them more.
1843		R.S. Surtees Handley Cross (1854) 345: Needs must when a certain old gentleman drives.
1850		G.G. Foster Clelio 94: If ye pacify Moll, ye’ll be the Ould Gintleman himelf, and nothing shorter.
1866		J. Greenwood Little Ragamuffin 259: The pleeceman [...] cut away as though the old gen’lman was arter him.
1873		Sl. Dict.
1888		W.B. Churchward Blackbirding In The South Pacific 76: Once I really thought I saw the ‘old gentleman’ in the middle of the flames.
1891		Sporting Times 3 Jan. 3/2: A barrel of prime natives, purchased [...] on the strict cash model, which was the old gentleman of it.
1895		C. Crowe Aus. Sl. Dict. 54: Old Gentleman, the devil.
1926		Wood & Goddard Dict. Amer. Sl.
1989		H. Rawson Dict. of Invective (1991) 277: Old Nick. One of many old nicknames for the devil, e.g., the Old Gentleman, Old Harry, Old Roger, and Old Scratch.

2. (also old gent) in gambling, a card that is slightly longer than the rest of the pack and thus identifiable by cheats (cf. old lady n. (1)).

1803		Sporting Mag. Feb. XXI 327/2: The Old gentleman – Is sometimes called a Brief Card [...] it being somewhat larger than any which are made up into packs; by which means, by fixing any card they think proper under it, they can always have the card so placed under it ready for a start.
1828		G. Smeeton Doings in London 40: A well-known macer, who is celebrated for slipping an ‘old gentleman’ (a long card) into the pack.
1831		Satirist (London) 20 Nov. 262/1: A well-known macer, who is celebrated for slipping an ‘old gentleman’ (a long card) into the pack.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict. 241: Gentleman [...] a card almost imperceptibly longer than the rest of the pack, used by sharpers for the purpose of cheating.
1935		A.J. Pollock Und. Speaks n.p.: Old gent, a playing card slightly longer than others of the pack.

3. the penis.

1902		D. St John Memoirs of Madge Buford 51: ‘When Ralph comes, I’ll make him give you a taste of his old gentleman’.
2011		L. Goodman on BBC Radio 4 Desert Island Discs 19 June 🌐 I’ll be naked [...] with my old gentleman flapping down between my legs.

4. (US) one’s father.

1911

E. Dyson ‘Nicholas Don and the Meek Almira’ in Benno and Some of the Push 17: What’s ah fellah to do, bai Jove, when the old gentleman says, ‘Go there, sir, and stay there, demmy, till you’re sent for’?