Ancient subjects kowtowed to the emperor as a sign of respect.
Tips
usage
叩 is more formal and literary than 敲 (qiāo, to knock). It appears in classical compounds: 叩头/叩首 (kòutóu/kòushǒu, to kowtow), 叩门 (kòumén, to knock on a door), and 叩拜 (kòubài, to bow and worship).