A mallrat is a young person who spends hours at a shopping mall to be with friends, not to buy things. They walk around, sit by the fountain, maybe purchase a cheap drink so nobody tells them to leave. It is informal American English, a bit jokey and a bit rude — as if the person lived at the mall like a rat. You would not use it in a serious school essay. It belongs to US mall culture of the 1980s and 1990s, so Taiwanese students may rarely hear it. Though actually, the “rat” is the same joke as in gym rat: not a thief, just someone who is always there. The word is about hanging out, not shopping.

整天泡在購物中心的年輕人（美式口語；台灣沒有對應的固定說法。不是真的「老鼠」，也不是去買東西）