¹ knock verb

• to hit something (such as a door) with the knuckles of your hand or with a hard object (such as a knocker) in order to get people's attention —usually + on —sometimes used figuratively
• to hit (something or someone) in a forceful way
• to touch or hit someone or something in a way that is not planned or intended

² knock noun

• a hard, sharp hit
• the sound made by a hard hit
• an experience that makes you less confident or successful for a period of time : a difficult or painful experience