Green’s Dictionary of Slang

1. in pl., the female breasts, esp. when unsupported but still firm.

c.1638		‘Praise of a Pretty Lasse’ in Chappell Roxburghe Ballads (1871) I 308: Her down Breasts are swingers indeed.
1750		Hist. of Jack Horner 18: The maid [...] came up in her smock, The sight he saw was a surprise, Perceiving such large swingers.
1984		Partridge DSUE (8th edn) 1188/2: since ca. 1930.

2. in pl., the testicles.

1968		G. Slatter Pagan Game (1969) 163: They had him by the swingers.
1984		Partridge DSUE (8th edn) 1189/1: C19–20.

3. (UK Und.) a lame leg.

1835		G. Kent Modern Flash Dict. 32: Swinger, one leg and a – a sound leg and a lame one.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].
1853		Bell’s Life in Sydney 23 Apr. 2/7: One good leg and a swinger.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 11: Swinger: Deformed leg.

4. (UK prison) a prisoner who has committed suicide by hanging.

2005

Guardian 15 Apr. 13/1: Harold Shipman [...] was found hanging in his cell. Andrew Belk [...] was told by a colleague they had a ‘swinger’ on the wing.