easy pickings
A person that is so weak, they let you tease them and laugh at them without even trying to fight back.
That kid in my class is such an easy pickings. I tease him all day and he doesn’t even try to fight back.
My little brother is an easy pickings. I poke him and he just laughs.
That girl in my group chat is an easy pickings. I made fun of her and she didn’t even say anything back.