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.
When I woke up in the morning, he was in the living room wearing nothing but his underpants.
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'.