A naval phrase meaning get out of your rack (bed). Derived from the days when women were allowed to accompany sailors to bed. If the privacy curtains were closed the order "show a leg" would instruct the person to put their leg outside the curtain so it could be determined if the person sleeping was a man (being a sailor) or his partner from the previous night.
The Chief inspected the sailor's berthing a noticed a rack with closed curtains and barked "Show a leg!". The sailor in the rack, who had overslept, quickly hopped out of his rack, threw on his uniform and spent the rest of the day cleaning heads.
by Drunken Sailor January 25, 2014