Green’s Dictionary of Slang

(US Mafia) a mistress.

1999		Observer Screen 1 Aug. 6: Goomah (sometimes pronounced ‘goomar’) a mistress.
2005		R. Cea No Lights, No Sirens 119: [T]here were some burnt-out denizens from the precinct [...] their goomadas, and the string of sexed-up Bettes from the Badlands.
2008		‘The Sopranos’ at hbo.com 🌐 Comare (also goomah, goomar, or gomatta): slang for girlfriend or mistress. No self-respecting wiseguy is without one.
2017		D. Winslow The Force [ebook] Donna Russo knows—she has to know—that her husband has a gumar on Columbus Avenue and gets strange every chance that comes up.
2020		(con. 1991-94) W. Boyle City of Margins 180: ‘He was sitting in his car, right?’ ‘Waiting on his goomar. She lived in Apartment 2C’.