Shake a leg
Definition 1 of 2
Pro Tip 1/3Urgency Expression
Used to express urgency, often when time is short or there's a rush. 
Shake a leg! Our train leaves in 10 minutes!
Pro Tip 2/3Informal Situations
Best used in casual or informal settings, not in formal situations. 
Shake a leg, kids! We don't want to miss the movie's start.
Pro Tip 3/3Mildly Imperative
It's a light-hearted command that pushes someone to act quickly. 
Hey, shake a leg! Everyone's waiting for you.