⺼ is the meat-flesh radical (visually identical to 月 moon but historically a slab of meat with two ribs inside). It marks 肌 as a body-tissue word, alongside 胸 (chest), 肝 (liver), 脸 (face), 脚 (foot). Always picture two horizontal ribs, not a moon.
几 (jī) gives the sound exactly - same initial, same rime, same tone. Same phonetic also fires 机 (machine), 饥 (hungry), 讥 (mock). Carries no semantic role here - purely a sound-marker that pins down jī among the meat-radical family.
