hit below (one's) weight

1. To perform, achieve, or do something at a level that is considered to be lower than one's abilities, talents, or personal attributes. The Oscar-winning actor is truly hitting below his weight in this new film, with a performance that is dull at best and downright amateurish at worst. I think she's scared to fail, and that's why she hits below her weight taking all these easy classes. Why would you hit below your weight and continue to work in the mail room? Come on, apply for this job upstairs—you're totally qualified!

2. To be romantically or sexually involved with someone who is considered by others to be inferior in physical appearance, intelligence, or success. It's odd to me that someone so motivated by his career would hit below his weight with some gas station attendant like her. It was fine to date this kid in high school, but he's on the road to nowhere, and you're going to college. It's time to stop hitting below your weight. She is stunningly gorgeous, so why is she hitting below her weight dating this nerd?