un ‧ der ‧ a ‧ chiev ‧ er /ˌʌndərəˈtʃiːvə $ -ər/ BrE AmE noun [countable]
[ Word Family: noun : ↑ achievement , ↑ achiever , ↑ underachiever ≠ OVERACHIEVER , ↑ underachievement ≠ OVERACHIEVEMENT ; verb : ↑ achieve , ↑ underachieve ≠ OVERACHIEVE ; adjective : ↑ achievable ]
someone who does not do as well at school or at work as they could do if they worked harder
—underachieve verb [intransitive]
—underachievement noun [uncountable]