Left mouth radical - the indexing component, doubling down on the meaning. 嘴 originally referred to a beak; adding 口 turned it explicitly into the human or animal mouth. Same family as 吃 (eat), 唱 (sing), 咬 (bite).
phonetic
觜zī
beak; horns
Right 觜 supplies both sound and meaning - it originally pictured horns or a bird's beak, and reads zuǐ in this position. The full 嘴 = mouth-radical clarifying a once-ambiguous beak/horn graph as the everyday word for mouth.
