Undersexed describes a person who wants sex less than is thought usual. It is an adjective about a person's level of desire, not about how often they actually have sex. Two honest things to say about this word. First, it is old-fashioned and rather judgemental: 'normal' here means whatever the speaker assumes is normal, and doctors today would talk about 'low libido' or 'low sex drive' instead, because those describe the level without implying something is broken. Second, in real speech it appears far more often as a joke or an insult than as a serious description — people say it about a grumpy colleague, or a character in a film, usually to be unkind. Its partner word 'oversexed' behaves the same way. So: understand it when you read it in a novel or hear it in a film, but for anything real — a doctor's visit, a serious piece of writing — say 'low sex drive' instead. Note also that it is a private, adult topic, and English speakers do not raise it casually with people they do not know well.

性慾偏低的。屬帶貶意的舊式說法；正經場合說「性慾低落」（low libido）。