To lip-sync is to silently mouth the words along with a song or some other recording. So grab a candlestick, crank the music, and lip-sync while you bust moves around the living room like Tom Cruise in "Risky Business."
Sync here is short for synchronization, from the Greek synkhronizein, "occur at the same time." Synchronization was first used in the movie biz, referring to getting the pictures and sound matched up. Still, actors often lip-sync — if it's done well, it looks like it's the actor who's singing. Bollywood films are almost always made this way. The best part of lip-syncing is the dance moves.