plain clothes
US /ˌpleɪn ˈkloʊðz/
UK /ˌpleɪn ˈkloʊðz/

1.
便衣, 常服
ordinary clothes, not a uniform or special costume
:
•
The detective was working in plain clothes.
侦探穿着便衣工作。
•
Security personnel often blend in plain clothes at events.
安保人员在活动中常穿便衣混入人群。