内裤 (nèi kù) literally means 'inner trousers' — 内 (nèi) means 'inside' or 'inner', and 裤 (kù) means 'trousers' or 'pants'. Together, they refer specifically to underpants: close-fitting underwear worn beneath outer clothing by people of all ages and genders. It is a neutral, standard term used in everyday speech, shopping, healthcare, and personal hygiene contexts.

This word is commonly used when discussing laundry, packing for travel, buying new clothes, or talking about health and comfort. Unlike some English terms, 内裤 carries no inherent formality or informality — it’s appropriate in both casual conversation and written instructions (e.g., on care labels). Note that while 裤 always refers to lower-body garments with two leg openings, 内裤 specifically denotes the innermost layer, distinguishing it from outer pants like 牛仔裤 (jeans) or 西裤 (dress trousers).