wearing nothing but underpants
Describes a state of undress where the person is wearing only a single pair of underpants, often used in casual or humorous contexts.
朝起きたら、彼はパンツ一枚でリビングにいた。
When I woke up in the morning, he was in the living room wearing nothing but his underpants.
パンツ一枚で外に出るのはやめてください。
Please don't go outside in just your underpants.
Compound of パンツ (underpants) and 一枚 (one thin flat object), literally 'one pair of underpants'.