腮

Left-side flesh radical - the side form of 肉, visually identical to 月 moon but historically distinct. Tags 腮 as a body part, the standard prefix for anatomy chars alongside 脸 face, 腿 leg, 胸 chest. The cheek is the fleshy side of the face, so the radical does direct semantic work here.

Right side 思 supplies the sound - sī drifting to sāi through a tone-and-vowel shift attested in this small phonetic family. 思 itself is the compound of 田 (originally a fontanelle graph) over 心 heart, depicting thought. No semantic connection to cheek - pure phonetic borrowing, meaning rides entirely on the flesh radical.