Green’s Dictionary of Slang

breakyleg n.1

also breake-leg, breaky
[ety. unknown; ? the price of breakyleg n.2 ]

a shilling (5p).

[UK]H. Brandon Dict. of the Flash or Cant Lang. 161/2: Breaky leg – a shilling.
[UK]J. Archbold Magistrate’s Assistant (3rd edn) 446: A shilling. Breake-leg.
[UK]Hotten Sl. Dict.
[UK]Derbyshire Courier 12 Dec. 7/1: Local Flash language [...] A breaky-leg, a shilling.
[Aus]Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: For a shilling there are many names but nearly all slang. [...] ‘Breaky-leg,’ ‘brongs,’ ‘bobs,’ ‘bordes,’ ‘drawers,’ ‘gens’, ‘hogs,’ levys,’ ‘pegs,’ ‘stags,’ ‘Shigs,’ ‘twelvers’ and ‘teviss’s’ .
[UK]Clarkson & Richardson Police! 320: A shilling ... A bob, breaky, deaner, midgic.
[UK]Exeter & Plymouth Gaz. 4 Feb. 5/6: A shilling was known as a ‘breaky-leg’.
[Aus](ref. to 1850s) Western Mail (Perth) 28 May 21/1: [from Daily Mail, London]At the time of the Crimean War bob was only one of a number of terms [for a shilling] such as twelver and breaky-leg, gen and teviss, stag, deaner, hog and levy.