膀 has multiple readings: bǎng = shoulder/wing (most common), pāng = swollen/puffy (e.g. 脸膀了 'face is puffy'), páng = used in 膀胱 (pángguāng, bladder), and bàng in the slang 吊膀子 (diào bàngzi, to flirt). Context decides.
memory
Radical 月 (flesh) + phonetic 旁 (páng → bǎng). 月 marks body parts; pair it with 旁 'side' and you get 'the thing on the side of your torso' — your shoulder/arm.
