Green’s Dictionary of Slang

1. (also jacko, jacks) the penis.

1604		Dekker Honest Whore Pt 1 I i: [He] taught her to play upon the Virginals, and still his Jacks leapt up.
1834		‘Parody on Love’s Ritornella’ Flash Chaunter 44: Your Jack you have slip’d / (Of time) in the nick.
1840		‘The Female Gamester’ in Facetious Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 271: Whene’er she deals she cribs or steals, / And can always lug out Jack.
1875		E. Sellon New Epicurean 21: Away flew too material buttons, and Jack sprang out of his box into her hand.
1882		Cremorne I 28: This fresh bit of fish made Jacko rise again.
1898		‘Mary Suckit’ Yvonne 24: Employing terms like «prick, Jack, rammer», the gentleman [...] took out his bursting cock.
1927		Immortalia 9: Pete war thar with every tack, / And kept a-lettin’ out more jack.
1945		in Randolph & Legman Ozark Folksongs and Folklore (1992) II 670: [as cit. 1927].

2. an erection; thus on jack adj., erect; thus often as sexual desire.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1959		personal recollection: on jack.
1963		A. Baron Lowlife (2001) 26: From too much staying in bed I got the jack up to my eyebrows.
1988		(con. 1940s) C. Bram Hold Tight (1990) 185: I [...] ’jaculate my jack / Into some seafood mama.
1989		J. Morton Lowspeak 78: Jack – erection, as in ‘I had a jack up to my eyebrows’.

3. copulation.

1984

Partridge DSUE (8th edn) 607: since ca. 1950.

4. an act of masturbation.

1984		C. White Life and Times of Little Richard 128: I haven’t had my morning jack yet!
2008		Simon & Burns ‘Bomb in the Garden’ Generation Kill ep. 7 [TV script] Takes me to this porn shop, so he can have a jack in the booths.

5. (US black) a gun.

1995

L. Stavsky et al. A2Z.