Plain-clothes describes police officers who are working in ordinary everyday clothes instead of uniform, so that people around them do not know they are police. A plain-clothes officer, plain-clothes detectives, a plain-clothes patrol. The point is not fashion — 'plain' here means 'not a uniform', so a plain-clothes officer might be in a suit, jeans, or a hoodie.

Plain-clothes這個詞 describes用來形容 police officers警察 who are working in ordinary everyday clothes instead of uniform穿一般日常服裝而不是穿制服執勤的，so that people around them do not know they are police好讓周圍的人不知道他們是警察。A plain-clothes officer, plain-clothes detectives, a plain-clothes patrol「plain-clothes officer」「plain-clothes detectives」「plain-clothes patrol」都是這樣用。The point is not fashion重點不在時尚——'plain' here means 'not a uniform'這裡的 plain 意思是「不是制服」，so a plain-clothes officer所以一個便衣警察 might be in a suit, jeans, or a hoodie可能穿西裝、牛仔褲、或連帽外套。

It is used before a noun, almost always, and almost always about police or sometimes soldiers and security staff. You can also drop the hyphen and use it as a phrase after a verb: 'the officers were in plain clothes', which is very common and perhaps the shape you will meet most. Hyphenated before the noun, two separate words after 'in' — that is the usual pattern, though writers are not perfectly consistent.

It is used before a noun它幾乎都用在名詞前面，almost always幾乎都 about police or sometimes soldiers and security staff是在講警察，有時候是軍人和保全人員。You can also drop the hyphen你也可以把連字號去掉 and use it as a phrase after a verb當成片語放在動詞後面：'the officers were in plain clothes'「the officers were in plain clothes」（這些警員當時穿便衣），which is very common and perhaps the shape you will meet most這種寫法很常見，可能是你最常遇到的形式。Hyphenated before the noun名詞前面加連字號，two separate words after 'in''in'後面分開寫兩個字——that is the usual pattern這是一般的規則，though writers are not perfectly consistent不過作者寫法不一定完全一致。

It is a neutral, factual word. It does not by itself suggest anything sneaky; news reports use it simply to explain why a witness did not realise the man was an officer.

It is a neutral, factual word這是一個中性、陳述事實的詞。It does not by itself suggest anything sneaky它本身並不帶有偷偷摸摸的意味；news reports use it simply to explain why a witness did not realise the man was an officer新聞報導用它只是要解釋為什麼目擊者當時沒發現那個人是警察。

'Undercover' is the stronger neighbour: an undercover officer is pretending to be someone else, living a false identity, while a plain-clothes officer is just not wearing the uniform.

'Undercover'「undercover」是語氣更強的近義詞：an undercover officer臥底警察 is pretending to be someone else假裝是別人，living a false identity過著假身分的生活，while a plain-clothes officer而便衣警察 is just not wearing the uniform只是沒穿制服而已。