to be careful in a situation that could be dangerous.
You have to watch your step when you're dealing with him, as he has an awful temper.
Related vocabulary: watch your back
idiomwatch your step(also mind your step BrE)1 to be careful about what you say or do, so that you do not cause problems for yourself:"We can't take any more risks." "Don't worry, I'll watch my step."Late again, Carly? You'd better watch your step or you'll be losing your job.2 spoken used in order to warn someone to be careful when they are walking, or they might hurt themselves:The adder is poisonous but it usually only bites in self defence - just watch your step!Mind your step, that railing's loose.