pick on (someone)
Definition: to treat someone who is less powerful than you badly and unfairly, often repeatedly.
(This phrasal verb has more than one meaning)
E.g.1: I hate that teacher. He’s always picking on me but I’m not the naughtiest student in the class.
E.g.2: His classmates picked on him because he was a little overweight.
This phrasal verb cannot be separated.