biàn​yī

plain clothes

便衣 biàn​yī
plain clothes noun
civilian or ordinary clothes worn instead of a uniform

He is wearing plain clothes today instead of his police uniform.

plainclothesman noun
a police officer or undercover agent wearing civilian clothes while on duty

Several plainclothes officers mingled with the crowd to carry out their mission.

Frequency

Movie subtitles
2.24 per million appears in 1% of films
Written text
3.39 per million

Sources

Definitions
CC-CEDICT