膀 is a bound morpheme (not normally used alone) that appears in compound words for body parts like 肩膀 (shoulder), 臂膀 (arm), and 翅膀 (wing). In modern Chinese you won't say just 膀 to mean 'upper arm'—that meaning is carried by the full compound 肩膀 or 上臂. Use it as a building block inside these common two-syllable nouns.