1. To be very careful in one's actions or speech, so as not to do or say something harmful or offensive. Knowing that Keith's family might not approve of her liberal beliefs, Janet reckoned she ought to watch herself at their barbecue that coming weekend. I'm so used to swearing with my friends that I really have to watch myself when I'm around my brother's kids.
2. imperative Be cautious of danger or possible injury. Watch yourself on the roads tonight, there are always a lot of crazy drivers out on New Year's Eve.
See also: watch