To pick on somebody is to treat one person unfairly again and again. Stop picking on him, always pick on the same boy.
The repetition is what makes it bullying rather than a single unkindness. Children use the phrase far more than adults, though the behaviour is not confined to them.
1phrasepick onTo treat one person unfairly again and again.
“They pick on the newest apprentice every year.”
pick on is a phrase.
Practised in Phrasal Verbs