膀子 (bǎng zi) is a colloquial, informal noun meaning 'upper arm'—specifically the part from the shoulder to the elbow. Though it contains the character 膀 (bǎng), which appears in formal terms like 肩膀 (jiān bǎng, 'shoulder') and refers to the shoulder or upper arm region, the addition of the diminutive suffix 子 (zi) makes 膀子 sound more casual and conversational. It’s commonly used in spoken Mandarin, especially in northern dialects and everyday speech, but rarely appears in formal writing or medical contexts.

This word emphasizes physical strength, movement, or bodily sensation—for example, when describing soreness, muscular effort, or visible muscle definition. Unlike the more neutral and standard term 上臂 (shàng bì), 膀子 carries a folksy, down-to-earth tone and often appears in idiomatic expressions or descriptive phrases involving exertion, labor, or physical appearance.