Green’s Dictionary of Slang

spare n.2

[SE spare]

1. a married man’s mistress, a wife’s lover.

[UK]Partridge DSUE (8th edn) 84/1: since ca. 1935.

2. (US black) a friend.

[US]M.H. Boulware Jive and Sl.

3. (also bit of spare) an unattached woman, considered to be open to male sexual advances; thus have a bit of spare v., to commit adultery.

[US]S. Longstreet Flesh Peddlers (1964) 284: ‘Anyway, I’m taking a girl along.’ Ira shrugged his shoulder. ‘Nothing like a spare.’.
[UK]D. Powis Signs of Crime 201: Spare, to have a bit of Sexual liaison outside marriage, adultery [...] Spare An unattached female.
[UK]J. Morton Lowspeak 131: Spare – a female available for extra-marital intercourse.

4. an idiot, a boring person.

[Ire]F. Mac Anna Last of the High Kings 122: ‘Don’t be such a spare,’ Hopper said.

5. an unattached person.

[UK]Indep. Rev. 18 Feb. 5: No one ever looks outside the square for a sexual partner, with the result that the most fabulously improbable pairings go on as any old pair of spares fall for each other.