punch below one's weight

• hit below one's weight

An allusion to the sport of boxing, where contenders fight in various weight classes.

punch below one's weight

1. (idiomatic) To achieve or perform at a level lower than should be expected based on one's preparation, attributes, rank, or past accomplishments.
   • 2001 Sept. 29, "For men who should know better," Telegraph (UK) (retrieved 23 Dec. 2009):

     But the overall effect is flimsy and insubstantial, a gifted writer punching well below his weight.

• underachieve

• overachieve, punch above one's weight