SOED: shake a leg (a) make a start; hurry up; (b) begin dancing.
Langenscheidt: shake a leg a) ugs. das Tanzbein schwingen, b) sl. ,Tempo machen'

http://www.phrases.org.uk/meanings/shake-a-le...

Shake a Leg - Meaning

Rouse yourself from sleep and get out of bed.

Origin

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."
[...]

'das Tanzbein schwingen' for 'shake a leg' hasn't died out completely, or so it seems.
My suggestion would be to add an 'obsolete'.

NOAD (2001):
shake - ... shake a leg - [informal] make a start; rouse oneself: come on, shake a leg.

AHD2 (1982):
shake - ... shake a leg. [Informal.] 1. To dance. 2. To move quickly; hurry up.

Random House unabridged (1966):
leg - ... 20. shake a leg, [Slang.] a. to hurry up. b. to dance: Let's step out on the dance floor and shake a leg.

Webster's 3rd unabridged (1961):
shake a leg - 1: DANCE {shaking a leg at junketings and fairs —Siegfried Sassoon} 2: to hurry up: move quickly {if you shake a leg and somebody doesn't get in ahead of you —John Dos Passos}

I too suspect that the 'dance' sense is essentially obsolete. I would be surprised if anyone could cite an example of it in current writing.

My older dictionaries still list that sense, but the newest one doesn't. I would be curious to see whether more recent dictionaries still list it, and if the online OED shows any recent citations for it.

Siegfried Sassoon was born in 1886, and IIRC did much of his writing several decades before he eventually died in 1967. That line actually sounds to me as if it might have had a historical context, referring to country people going to 'junketings' in some quaint, idyllic past time.

It's clear that the writer at phrases.org.uk considered the sense obsolete, since he or she uses the past tense to refer to it: 'Before that[,] though[,] ["]shake a leg["] had another meaning.'

http://www.guardian.co.uk/friday_review/story...
"Behind him, an industrial-sized head of steam stokes up and Edge's guitars howl like the backdraft from a low-flying 747. To demonstrate that the mature rock'n'roller can still shake a leg and blow out a few speakers, they've spliced together an irresistible mix of crude techno and raw guitar-swagger in Elevation, the closest to leathers-and-grunge that U2 have come in many a year. "

http://www.guardian.co.uk/stage/2010/jul/12/b...
"Big Dance takes steps towards the London Olympics
...At one in Highbury Fields the Sunday before, an instructor called Vicky encouraged north Londoners (mostly the very old and the very young, plus a few hectically sweaty joggers) to shake a leg."

http://www.guardian.co.uk/lifeandstyle/2009/j...
"Celebrate Independence Day by heading to the American Museum in Britain where today's shenanigans include crown forces drilling, jive lessons and classic 50s rock to shake a leg to."

self-consciously twee and old-fashioned, perhaps, but I really wouldn't call it obsolete.

All quotes above are from the first page of Google hits on site:.guardian.co.uk, so I am sure I could find many more at the Independent, the Times, the BBC, etc.

It's a bit like "shenanigans", I think - old-fashioned, but definitely not obsoloete.

edit: and, in fact, a bit like "das Tanzbein schwingen" - you'd only apply that to The Edge from U2 for comic effect, not because it's a proper, up-to-the-minute equivalent of "tanzen"...

Gosh, now I really am surprised; I wouldn't expect to see quotes like that in American media. (Where 'shenanigans,' by the way, are still a perfectly current description, and entirely apt for the recent campaign.)

I wonder if it's less obsolete in BE?

At any rate I'm all agog for the new usage marking

[self-consciously twee and old-fashioned]

in LEO. (-;

For completeness, the cross-reference to the previous discussion:

related discussion: to shake a leg - das Tanzbein schwingen