to smush: to mash or push, especially to push down or in; compress:
ex: To smush a pie in someone's face.
Smush up has a similar meaning to smashed up, which means a collision (moslty serious, like between vehicles). In this sentence the narrator asks if you've ever worn a pair of pants and then noticed that there was a loose sock (uma meia solta (perdida entre as outras roupas) that then got smushed up (copresses/pushed) against your thight by the pants. Hope it helps