Left hand radical - the side-stacking variant of 手. Anchors 扣 in hand-action verbs: fastening a button, knocking on a door, clasping a buckle, deducting (pulling money back). All are concrete things hands do to small objects.
Right 口 supplies the sound - kǒu shifted only in tone to kòu (3rd → 4th). 口 also offers a faint pictorial nudge: a small opening or hole, fitting for a button-hole or buckle-loop. Same phonetic family: 叩 (knock - same meaning), 寇 (bandit).
