扣 is versatile: 扣分 (deduct points), 扣工资 (deduct from salary), 扣子/纽扣 (button), 扣篮 (slam dunk in basketball). 打折扣 (dǎ zhékòu) means 'to give a discount.'
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 (kòu, knock — same meaning), 寇 (kòu, bandit).
