Green’s Dictionary of Slang

show a leg! excl.

[orig. used in institutions to ensure that the leg was masculine and not, illicitly, female; note Fraser & Gibbons Soldier & Sailor Words & Phrases (1925): ‘In the Navy, it dates from long ago, when women, ostensibly as sailors’ wives, were allowed to live on board ship. The usual call was, ‘Show a leg or a purser’s stocking!’ Everybody had to put a leg outside the hammock, a stockinged leg denoting that its occupant was a woman, who was then allowed to remain until the men had cleared out’]

a wake-up call.

Show quotations
[UK]‘A. Burton’ Adventures of Johnny Newcome I 47: ‘Hoy! shew a leg, and save a clue!—’ Rouse! rouse!—heave out!
[UK]‘Cuthbert Bede’ Adventures of Mr Verdant Green (1982) II 198: You must show a leg, sir!
[UK]W.H. Smyth Sailor’s Word-Bk (1991) 622: Show a Leg! [...] Often ‘Show a leg, and turn out.’.
[UK]J.T. Keane On Blue-Water 153: Tumble out here, all hands; show a leg.
[UK]Boy’s Own Paper 13 Apr. 437: Suddenly a gruff voice roared through the darkness to ‘Show a leg there,’ and I awoke.
[US]Denton (MD) Journal 24 Oct. 1/7: Slang of the Sailor [...] The duty of calling the men in the morning falls to the master at arms, and he says ‘show a leg’ or ‘rise up and shine’.
[UK]‘Bartimeus’ ‘The Night Watches’ in Naval Occasions 181: ’Eave out, ’eave out, ’eave out ! Show a leg there, show a leg ! Sun’s a -scorching your eyes out!
[UK]Marvel 1 Mar. 16: Show a leg, you cannibal freaks!
[UK](con. WWI) Fraser & Gibbons Soldier and Sailor Words 142: Leg, Show A: Wake up. Turn out.
[UK]J. Hanley Boy 142: Show a leg, you sucker.
[Aus]T. Wood Cobbers 79: Show a leg, me lucky lads!
[UK] ‘Kiss Me Goodnight, Sergeant Major’ in M. Page Kiss Me Goodnight, Sgt.-Major (1973) 26: When we hear you calling – ‘Show a leg!’ / Don’t forget to wake me in the morning / And bring me a nice cup of tea.
[US]P. Kendall Dict. Service Sl. n.p.: show a leg . . . arise from your hammocks, merry gentlemen, let nothing you delay.
[UK]C. Harris Death of a Barrow Boy 53: Wakey-wakey! [...] Show a leg there! Rise and shine!
[UK]G. Melly Rum, Bum and Concertina (1978) 27: The traditional cry ‘Show a leg’ derives from the sexually permissive eighteenth century when sailors were allowed to cohabit with women between decks.