Ordinary/casual clothes (not a uniform); private clothes.
今日は私服で出勤していいと言われた。
I was told I could come to work in plain clothes today.
面接なのに私服で来るのは避けたほうがいい。
It's better to avoid coming to an interview in casual clothes.
教師でも授業のときは私服の先生もいる。
Some teachers teach classes in plain clothes.
Abbreviation referring to a plainclothes police officer (a detective not in uniform).
あの人、私服の刑事かもしれない。
That person might be a plainclothes detective.
事件現場には私服の刑事が張り込んでいた。
Plainclothes detectives were staking out at the crime scene.
私服での捜査は目立たないのが利点だ。
An advantage of undercover/plainclothes investigations is that they attract less attention.