Green’s Dictionary of Slang

the number eleven; elevenpence.

1859		Hotten Dict. of Modern Sl. etc. 127: LEVEN, in back slang, is sometimes allowed to stand for eleven, for the reason that it is a number which seldom occurs. An article is either 10d. or 1s.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 23: Leven ... Elevenpence.
1870		Hotten Sl. Dict. [as cit. 1859].