a pick
When you treat someone like a piece of trash just because you're mad at them. You nag them, berate them, and make their life a living hell for no good reason.
My mom picked on me for not doing my homework. She even took my snack.
My teacher picked on me for doodling in class. I had to write 100 times 'I will not doodle in class.'
My friend picked on my dog for eating my lunch. The dog was confused.