One who goes to a mall with no shopping agenda
Mallrats usually know everything that's going around in the mall (I,e: when the Easter bunny stand comes up) and knows about everything about the mall (what stores going where how long a store has been there and what store was there before that store)
also a kick ass movie by Kevin Smith
"You're one of those loser, fucking MALLRAT kids. You don't come here to Work or Shop -- You hang out and act like you fucking live here."