1
general::
Tease, bully, victimize, as in She told Mom the boys were always picking on her. [Second half of 1800s] This expression is sometimes put as pick on someone your own size, meaning "don't badger someone who is younger, smaller, or weaker than yourself but do so only to an equal."
American Heritage Idioms
2
general::
Phrase(s): pick on someone or something
to harass or bother someone or something, usually unfairly. • Please stop picking on me! I’m tired of it. • You shouldn’t pick on the cat.
McGrawhill's American Idioms And Phrasal Verbs