(as) skinny as a beanpole
Extremely thin. I was always skinny as a beanpole growing up, but I really filled out once I hit puberty. You want to play football? But you're as skinny as a beanpole—you'll get creamed!
skinny as a beanpole
very thin; very skinny. (*Also: as ~.) I exercised and dieted until I was skinny as a beanpole.