plain clothes

n. 便衣, 便服

例句与用法：

Plain clothes are appropriate for school wear.

朴素的衣服适合于上学时穿着。

Policemen in plain clothes intermingled with the crowd to prevent trouble during the foreign ruler's state visit.

便衣警察在这位外国首脑进行国事访问期间混入在人群中以防骚乱。

A policeman in plain clothes shadowed the criminal all day.

有个便衣警察整天秘密监视那个罪犯。