Green’s Dictionary of Slang

(orig. US) a cheat, a double-crosser.

1928		Bessie Smith ‘Devil’s Gonna Git You’ 🎵 Dirty two-timer, dirty two-timer, / Dirty two-timer, you ain’t coming clean.
1933		R. Chandler ‘Blackmailers Don’t Shoot’ in Red Wind (1946) 101: I think we’ll give it to you right here, two-timer.
1943		C. Thompson These My People 21: He had [...] never heard of Liza Ann being a two-timer.
1956		T. Thursday ‘The Big Squawk’ in Smashing Detective Mag. 15 Apr. 🌐 So you’re not only a two-timer, you admit you are a three-timer.
1967–8		Baker et al. CUSS 215: Two timer A person who always fools around.
1988		D.D. Harrison Black Pearls 123: [W]e hear [in a song] a wronged woman who [...] ushers the mistreating two-timer out of her house and out of her life.
2001		N. Barlay Hooky Gear 183: Now I feel like a 2-timer cos I’m just provin to Monica wha I’m like.