If you tell someone to beware of a person or thing, you are warning them that the person or thing may harm them or be dangerous. Beware of being too impatient with others Beware, this recipe is not for slimmers. used to warn someone to be careful because something is dangerous beware of (be + ware (11-19 centuries) (from wAr))
be on one's guard; be cautious or wary about; be alert to; "Beware of telephone salesmen"