An undershirt is a piece of underwear worn against the skin, under your shirt. It may have short sleeves or no sleeves at all. Its job is to keep you warm, to soak up sweat so your outer shirt stays clean, and to stop a thin shirt from showing your skin.

An undershirt is a piece of underwear worn against the skinundershirt是貼身穿的一種內衣, under your shirt穿在襯衫裡面。It may have short sleeves or no sleeves at all可能有短袖，也可能完全無袖。Its job is to keep you warm它的作用是保暖, to soak up sweat so your outer shirt stays clean吸汗，讓外面的襯衫保持乾淨, and to stop a thin shirt from showing your skin還有避免薄襯衫透出皮膚。

The important thing for you to know is that this is the American word. British English calls the same thing a vest — which causes real confusion, because in American English a vest is the sleeveless thing you wear OVER a shirt as part of a three-piece suit (British: waistcoat). So one word, two opposite layers, depending on which side of the Atlantic you are on.

The important thing for you to know is你需要知道的重點是 that this is the American word這是美式的說法。British English calls the same thing a vest英式英文把同樣的東西叫做vest — which causes real confusion這會造成真正的混淆, because in American English a vest is the sleeveless thing you wear OVER a shirt因為在美式英文裡，vest是穿在襯衫外面的無袖背心 as part of a three-piece suit是三件式西裝的一部分 (British: waistcoat)（英式叫waistcoat）。So one word, two opposite layers所以同一個字，卻是兩種相反的層次, depending on which side of the Atlantic you are on要看你在大西洋的哪一邊。

In Taiwan you will hear 'undershirt' more often, because American English dominates in schools here. Everyday speakers also have nicknames: a plain white sleeveless one is called a tank top (US) or, rudely, a 'wife-beater' — that last one is genuinely offensive and I would not use it.

In Taiwan you will hear 'undershirt' more often在台灣你比較常聽到『undershirt』, because American English dominates in schools here因為這裡的學校主要教美式英文。Everyday speakers also have nicknames一般人講話也有暱稱: a plain white sleeveless one is called a tank top (US)純白無袖的那種美式叫tank top, or, rudely, a 'wife-beater'或者不禮貌地叫『wife-beater』 — that last one is genuinely offensive and I would not use it最後這個講法真的很冒犯，我不會用它。

Note the spelling is one word, no hyphen.

Note the spelling is one word注意拼法是連在一起的一個字, no hyphen沒有連字號。