An underachiever is a person who does less well than their ability suggests they could. Teachers use it about the clever student whose homework never arrives, and it carries a particular flavour: not stupid, not incapable, just not delivering. Outside school it stretches to work and sport, and people also use it half-jokingly about themselves — 'I'm a professional underachiever' — which softens it into something almost fond. Said about someone else, though, it can sting, because it implies both that they had the talent and that they wasted it. It is countable, so it takes a and the plural: an underachiever, several underachievers. Its opposite, overachiever, is not really the same shape of word — that one means doing better than expected, and it is often said with a mix of admiration and teasing.

低成就者、沒發揮實力的人（明明有能力卻表現不佳的人）