- 1便衣的（警察不穿制服執勤）
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. It is used before a noun, almost always, and almost always about police or sometimes soldiers and security staff. You can also drop the 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. before the noun, two separate words after 'in' — that is the usual pattern, though writers are not perfectly consistent. 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. '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.
plain-clothes 指警察（有時也指軍人或保全）穿一般便服執勤，不穿制服。名詞前通常加連字號，像「a plain-clothes officer」、「a plain-clothes patrol」；放在 in 後面則分開寫，例如「the officers were in plain clothes」。重點是沒穿制服，所以西裝、牛仔褲或帽T都算。它是中性、陳述事實的字，新聞常用來解釋目擊者為什麼沒看出對方是警察。假身分、臥底的人則是 undercover，那是過著假的生活，便衣只是換掉制服。
組成plain ('ordinary, not decorated, not special') + clothes. So: wearing ordinary clothes rather than the uniform that would mark you out.
同家族的字
- plainthe everyday adjective for 'simple, ordinary' — the same word, used here to mean 'not a uniform'.
原始詞典資料1 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
這個拼法也被記錄為下列字的變化形：
- plainclothes
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。