膊 (bó) is a bound form (b.f.) meaning 'arm'. In modern Chinese it almost always appears in the compound 胳膊 (gēbo) for 'arm'. You won't use 膊 alone as a standalone word; the common expression is 光着胳膊 (bare-armed) or 胳膊肘 (elbow). The bound status means 膊 only surfaces in fixed words and the classifier 一条胳膊 works for counting arms.