Green’s Dictionary of Slang

spare n.2

[SE spare]

1. [1930s] a married man’s mistress, a wife’s lover.

2. [1940s] (US black) a friend.

3. [1960s+] (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.

4. [1990s+] an idiot, a boring person.

5. [2000s] an unattached person.