Shake a leg
Rouse yourself from sleep and get out of bed.
Shake a leg and show a leg are usually discussed together, but it isn't at all clear that they are in any way connected. Both phrases have more than one meaning. So what were their original meanings and which came first?
We now sometimes use shake a leg to mean 'hurry up'. It was explicitly defined that way in the New York Magazine in 1904:
"Shake a leg ... meaning to 'hurry up'."
The more recent UK phrase 'get a legger on' is another way of saying the same thing. Before that though shake a leg had another meaning, which was 'to dance'. There are several citations from various US and UK sources from the mid 19th century that relate to dancing; for example, the Dubuque Democratic Herald, October 1863, in an advertisement for a local ball:
"Nearly every man in town able to shake a leg has purchased a ticket."
Show a leg means either 'make an appearance', usually by getting out of bed or at least showing willing by poking your leg out, or it means 'hurry along'. The second meaning isn't commonly used, nor is it old. It appears to be a confusion of the two terms 'show a clean pair of heels' and 'stretch your legs'. It may also have been confused with the 'hurry up' version of shake a leg. Whatever the source, that certainly isn't the original meaning of show a leg. Most commentators report that the phrase derives from the Royal Navy and that this was the order given to sailors to put a foot from their hammocks and get up.
An alternative version comes from the fact that women were allowed on board Royal Navy ships in the 19th century and that they were allowed to stay asleep after the sailors had been roused. The order of show a leg was supposed to have been given so that the shapely legged women could be distinguished from the hairy-legged sailors. Believe that if you will; personally, I don't.
The use of show a leg as a wake-up call is well documented though. John Masefield (Poet Laureate from 1930 to 1967) was a trainee mariner on HMS Conway until 1891. He reported the full version of the morning call as:
Heave out, heave out, heave out, heave out! Away!
Come all you sleepers, Hey!
Show a leg and put a stocking in it.
(No connection there to put a sock in it by the way. That has a different meaning and is 20th century)
That's the earliest citation of the naval call I can find, although it may have been used well before 1891. There is an earlier non-naval version, from Cuthbert Bede, the pseudonym of Edward Bradley, in The Adventures of Mr. Verdant Green, 1854:
"I would answer Robert when he hammered at the door; but, instead of getting up, I would knock my boots against the floor... But that wretch of a Robert was too old a bird to be caught with this dodge; so he used to sing out, 'You must show a leg, sir!' and he kept on hammering at the door till I did."
All in all, although both are sometimes used to mean 'hurry up', the idea that shake a leg and show a leg are related doesn't seem to be supported by the facts. They are two independent phrases that were coined with different meanings.
See other Nautical Phrases.