Green’s Dictionary of Slang

a girlfriend, the antonym of blower n.¹

1839		H. Brandon Dict. of the Flash or Cant Lang. 163/2: Jomer – a fancy girl, – term of friendship, as opposed to blower.
1847		G.M.W. Reynolds Mysteries of London III 66/1: His jomen [sic] Mutton-Face Sal [...] stalled.
1852		Kendal Mercury 3 Apr. 6/2: A spicey moll [...] to whom he swears [...] ‘that should she think proper to become his “jo-man” ’ (left handed wife), she shall never be minus ‘the first scran (food) on the pad, and plenty of lush (drink) when she wants it ’.
1852		Kendal Mercury 3 Apr. 6/2: Should she think proper to become his ‘joman’ (left-handed wife), she shall never be left minus ‘the first scran (food) on the pad, and plenty of lush (drink) when she wants it’.
1857		‘Ducange Anglicus’ Vulgar Tongue 38: I buzzed a bloak and a shakester of a reader and a skin. My jomer stalled.
1862		W.C. Glen Snowden’s [...] Magistrates Assistant 107: His ‘jomer’ or fancy girl was also tried.
1874		Derbyshire Courier 12 Dec. 7/1: Local Flash language [...] jomer — a fancy girl.
1881		Newcastle Courant 16 Sept. 6/5: Don’t know where Nat and Sam are, but Sam’s jomer is sliding to —.
1882		Sydney Sl. Dict. 9/2: I pinched a swell of a fawney and fenced it for a double finnip and a cooter. My jomer stalled. I robbed a gentleman of a ring and sold it for a ten-pound note and a sovereign. My girl watched.
1889		Clarkson & Richardson Police! 320: A woman ... A molt, finger, smilto, stuffy, Jomer.
1895		C. Crowe Aus. Sl. Dict. 40: Jomer, a mistress; a female companion of thieves.