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.
Left meat-flesh radical — the side form of 肉, indexing 膀 as a body-part word. Visually identical to 月 (moon) but semantically distinct: 月-meat anchors anatomy chars like 胸 chest, 背 back, 脏 organ, 脚 foot, 肝 liver, 脸 face. Marks 膀 as a part of the human body — the shoulder or upper arm.
Right 旁 supplies the sound — páng shifting to bǎng with consonant change. 旁 means 'side / beside,' which also fits the meaning beautifully: the shoulder is the body part to your side. Mnemonic and etymology aligned. Same phonetic appears in 榜 list, 谤 slander, 螃 (蟹) crab. One of the cleaner phono-semantic pairs in the anatomy lexicon.
