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Related to knock on: Knock on Wood
knock on (something)
1. To strike something sharply and swiftly. I knocked on the door several times, but no one answered. The teacher began knocking on the desk to get the students' attention.
2. To strike someone sharply and swiftly on a particular part of their body. In this usage, a noun or pronoun is used between "knock" and "on." She opened the car door for me, but I wasn't looking and it knocked me on the shoulder. That book knocked me on the head when it fell off the shelf.
3. To bump into or collide with someone or something and cause them or it to fall down onto something, especially the floor or the buttocks. In this usage, a noun or pronoun is used between "knock" and "on." The thief rushed through the store with the stolen goods, knocking the security guard on his butt on his way out the door. The detective knocked the lamp on the ground in a fit of anger.
knock on something
to rap or tap, often with the knuckles, on something. (See also knock on wood.) She knocked on the door several times. Knock on it again. Maybe she didn't hear you.