"Shake a Leg" is an informal slang phrase that means to hurry up or get moving. If you're looking for synonyms to replace the phrase, you might try using "get a move on", "hurry up", "speed up", "quickly", "expedite", "rally", "hustle", "jump to it", "snap to it", "get cracking" and many more. All of these phrases convey a sense of urgency and promptness. Using such synonyms can add variety to your language and make your communication more colorful and expressive. So, go ahead and shake things up!