Green’s Dictionary of Slang

1. an adulterer, also attrib.

1916		J. Lait ‘Taxi, Mister!’ in Beef, Iron and Wine (1917) 139: He will have the concentrated shady knowledge of all the bloods, pikers, come-ons, roisterers, gamblers, cheaters, beaux, rich men’s sons, and poor men’s daughters.
1952		Lait & Mortimer USA Confidential 96: All it has to offer is whores. They are undergrade, as the more choosy cheaters skip to Boston.
1960, 1975		Wentworth & Flexner DAS.
1989		H. Rawson Dict. of Invective (1991) 77: cheater, a rearview mirror as well as one who is unfaithful sexually.
2023		(con. 1962) J. Ellroy Enchanters 82: Norm’s Nest [...] a dingy cheater bar in north Van Nuys.

2. (US) a glass eye.

1920

A. Baer Putting ’Em Over 2 Oct. [synd. col.] One-Eyed Connolly used to take out his glass hedadlight when he horned into a barroom scuffle [...] He unhooked his porcelain cheater and handed it to Honest Joe Spivins.

3. (US) an act of adultery.

1927

Immortalia 71: Nothing could be sweeter than to have a little cheater / In the morning.

4. a condom.

1946

in G. Legman Limerick (1953) 196: There was a young fellow named Peter / Who was laying his gal with a cheater, / When the rubber thing broke.

5. anything that makes a task simpler, provides safety, gives one advantage, etc.

1967–8

Baker et al. CUSS 95: Cheater Commercially produced subject outline.

6. see cheator n.