扣
koù · HSK 6
button
🔘 Hand (扌) + opening (口) = a hand fastening a button over an opening
口 alone is kǒu (mouth/opening). In 扣 the tone shifts to koù but the sound stays — your hand pushes a button through the opening to close it.
扣 — hand radical 扌 + 口 (mouth/phonetic)
扣 is a phono-semantic compound: the hand radical 扌 signals an action done with the hand, while 口 (kǒu) supplies the sound. Its core sense is to knock, fasten, or buckle with the hand, later extending to deduct (as in deducting points) and to button up.