Green’s Dictionary of Slang

1. (bingo) the number 11.

1917		A.G. Empey Over the Top 148: The caller-out has many nicknames for the numbers such as ‘Kelly’s Eye’ for one, ‘Leg’s Eleven’ for eleven.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 122: 11, ‘Legs Eleven’.
1936		(con. 1900s) F. Richards Old Soldier Sahib (1965) 70: No. 11 – Legs Eleven. (The number resembles a pair of legs, and was given this extra syllable to distinguish it from Seven and avoid mistakes).
1945		J. Henderson Gunner Inglorious (1974) 106: Eyes down! And the first number [...] legs Eleven.
1962		K. Waterhouse There is a Happy Land (1964) 88: Legs eleven. Sixty-five, old age pension.
1972		(con. 1940s) N. Conway Bloods 115: The caller had sung out his litany of Legs Eleven, Kelly’s Eye, Knock on the Door, Clickety-Clicks.
1980		T. McClenaghan Submariners I ii: tannoy: Legs eleven. Everyone wolf whistles.
2003		Observer 6 July 16: Old favourites such as [...] ‘legs eleven’, are staying put.

2. (Aus.) a tall thin man.

1919		W.H. Downing Digger Dialects 32: legs eleven [...] (2) a tall, thin man.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: legs eleven. [...] a thin tall man.

3. £11.

1989

(con. 1950s–60s) in G. Tremlett Little Legs 195: legs eleven £11.