beware of (someone or something)
Be cautious or mindful of something or someone, especially something or someone that might pose a danger of some kind. Beware of the boss today—he's been yelling at everyone he sees. Beware of their dog, he's vicious!
beware of someone or something
to be cautious and watchful about someone or something. Beware of Ted. He's acting irrational. You should beware of the dog.