Poorer than expected performance (poorer than might have been predicted from intelligence tests)

Usage examples of underachievement.

But I was at a dead end in my life, sterile and stale from overwork and underachievement, and I paid no attention to anything that took place beyond the confines of my own skull.

To most people, especially to Washington careerists, the idea of being a middle or low-ranking diplomat in a place like Sierra Leone would represent the ultimate in underachievement, unless it came very early in one’s career.

Frustrated at the boy’s chronic underachievement but explains it away as the school’s failure: not stimulating poor Andrew.