叹

Mouth radical on the left — the indexing radical and the obvious anchor for any verb of vocal expression. Sighs come out of the mouth; the radical places 叹 alongside 喊 (shout), 唱 (sing), 哭 (cry), 吵 (quarrel). Carries the meaning, not the sound.

Right side 又 'hand' — a simplified replacement for the more elaborate 莫/嘆 phonetic in the traditional form 嘆. The 1956 reform substituted this two-stroke hand to ease writing, abandoning the original phonetic link. Today it just signals 'a hand by a mouth,' the small gesture of clutching the chest as you sigh.