Green’s Dictionary of Slang

(UK Und.) one’s mistress of many years or one’s wife, but not one’s actual mother.

1930		K.S. Prichard Haxby’s Circus 187: And George, hugging his wife, exclaimed: ‘Lord, mum, if it weren’t for you, I’d be gone a million on the minx’.
1977		D. Powis Signs of Crime 194: Mum One’s wife or long-established mistress, and not one’s mother, unless it is plainly made to sound so by the addition of such words as ‘my dear old mum’.
1982		J. Davis Kullark 53: Me an’ Mum an’ the kids are gonna give those trackers the run of their lives.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 120/2: mum n. an inmate’s wife or girlfriend.