Green’s Dictionary of Slang

1. (UK Und.) six months’ imprisonment [horseracing imagery, a length is half a stretch n. (2a)].

1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1895		C. Crowe Aus. Sl. Dict. 44: Length, six months’ imprisonment.

2. the penis, usu. by metonymy an act of sexual intercourse; usu. with v., give a length, throw a length etc. [i.e. the ‘length’ of the penis].

1870		Cythera’s Hymnal 11: For a length he kept his strength / And did his duty.
1947		K. Amis letter 12 Sept. in Leader (2000) 141: Didn’t I tell you how I once refrained from slapping a length on Noel because it would hurt Hilly if I did?
1998		I. Welsh Filth 274: Any hoor gitting that length ay Gus’s fuckin well would [smile]. [Ibid.] 302: She’s long overdue a length.
1999		E. George ‘I, Richard’ Evidence Exposed (1999) 90: He’d make a feeble attempt to give her a length now and again if the mood was upon him.
2003		McGill Reed Dict. of N.Z. Sl. 127: length The penis, particularly when tumescent, eg, ‘Get your length in last night, Jock?’ ANZ.
2004		P. Howard PS, I Scored the Bridesmaids 222: He looks at me like he’s walked in and caught me giving his housekeeper a length. [Ibid.] 232: I’d throw Linda a length, actually.
2012		(con. 1980s) I. Welsh Skagboys 207: The poor cows’ve nae chance ay getting a length in that company.