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.
