Underweight means weighing less than you should. It is used most often about people and about health: a doctor says a baby, a patient, a teenager is underweight, meaning their weight is below the healthy range for their age and height. That is the main use, and it is a medical, neutral word — the doctor is not scolding anyone, just reporting a number.

Underweight meansUnderweight的意思是 weighing less than you should體重比應該要有的還輕 It is used most often about people and about health它最常用在人和健康方面 a doctor says a baby, a patient, a teenager is underweight醫生會說一個嬰兒、病人、青少年underweight meaning their weight is below the healthy range for their age and height意思是他們的體重低於這個年紀和身高該有的健康範圍 That is the main use這是最主要的用法 and it is a medical, neutral word而且是個中性的醫療用字 the doctor is not scolding anyone, just reporting a number醫生不是在罵人，只是在報告一個數字

It also works for things: an underweight parcel, an underweight bag of rice, meaning it does not reach the weight it is supposed to be.

It also works for things它也可以用在物品上 an underweight parcel, an underweight bag of ricean underweight parcel（重量不足的包裹）、an underweight bag of rice（不足重的一袋米） meaning it does not reach the weight it is supposed to be意思是沒有達到應該要有的重量

Two things to notice. First, it is an adjective, and it comes after the verb far more often than before the noun — "he is underweight" is normal English; "an underweight child" is fine too but a little more written.

Two things to notice有兩件事要注意 First第一 it is an adjective它是形容詞 and it comes after the verb far more often than before the noun出現在動詞後面的頻率遠高於放在名詞前面 "he is underweight" is normal English「he is underweight」是很正常的英文 "an underweight child" is fine too but a little more written「an underweight child」也可以，但比較偏書面

Second, it is genuinely neutral in a way that its opposite is not: "overweight" carries social weight and can hurt, while "underweight" mostly sounds like a health concern. Though actually, that is changing a little — because being thin is admired in Taiwan and elsewhere, some young people hear "you're underweight" as a compliment, which is not at all how a doctor means it.

Second第二 it is genuinely neutral in a way that its opposite is not它是真正中性的字，這一點跟它的反義詞不一樣 "overweight" carries social weight and can hurt「overweight」帶有社會壓力，可能會傷人 while "underweight" mostly sounds like a health concern而「underweight」聽起來大多只是健康上的擔憂 Though actually不過其實 that is changing a little這個情況正在慢慢改變 because being thin is admired in Taiwan and elsewhere因為在台灣和其他地方，瘦被認為是好看的 some young people hear "you're underweight" as a compliment有些年輕人會把「you're underweight」聽成是稱讚 which is not at all how a doctor means it但這完全不是醫生說這句話的意思

There is also a noun, "underweight", but you will almost never meet it outside medical writing.

There is also a noun, "underweight"還有一個名詞形式「underweight」 but you will almost never meet it outside medical writing但除了醫療寫作之外，你幾乎不會遇到它