To crush, crumple, or mash something into a broken or untidy state. A noun or pronoun can be used between "squash" and "up." Don't just squash your clothes up like that while you're packing—everything will be wrinkly when you unpack it later! The bully came over and squashed up my science fair diorama.
squash someone or something up
to grind someone or something up; to mash someone or something up. You had better stay out of the traffic, or some big truck will squash you up! The truck squashed up the tiny car.