腕 is the literary/formal form. In daily speech, 手腕 (shǒuwàn) is far more common for 'wrist'. 腕 alone appears mainly in compounds like 手腕 or 腕表 (wristwatch).
The meat-flesh radical on the left (a side-form of 肉, distinct from moon-月) marks 腕 as a body part. It places the wrist with 腿 leg, 胸 chest, 脸 face and the wider anatomy family — the joint of flesh between hand and arm.
Lends the sound, with only a tonal shift from wǎn to wàn. The winding meaning fits unexpectedly well: the wrist is the joint that lets the hand bend and curve in any direction, which makes 宛 a memorable phonetic pick.
