Watch out! A warning shouted in times of danger or to indicate one's presence behind someone. I yelled, "Behind you!" as soon as I saw the masked man reach for the unsuspecting woman's purse. All chefs know to say "behind you" when they're passing other chefs on the line.
Look behind you!; There is danger behind you! "Behind you!" shouted Tom just as a car raced past and nearly knocked Mary over. Alice shouted, "Behind you!" just as the pickpocket made off with Fred's wallet.