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).
Left mouth radical - the indexing radical. Pictures an open square. Here it sets 叩 in the action-with-voice/head family: knocking on a door is paired with calling out, and the original sense includes kowtowing 叩头, where the head touches the ground with audible greeting. Vocal accompaniment is built in.
Right kneeling-person radical - pictures someone bowed at the knees. Reinforces meaning rather than sound: a compound ideograph 'mouth + kneeling' captures 叩头, the reverent kowtow with forehead striking the ground. By metaphor the same action gave 'to knock' - striking a surface deferentially as on a door.
