Read the dictionary definition of knock. All definitions for this word.
1. the act of hitting vigorously
1. he gave the table a whack
2. negative criticism
3. a bad experience
1. the school of hard knocks
4. the sound of knocking (as on a door or in an engine or bearing)
1. the knocking grew louder
5. a vigorous blow
1. the sudden knock floored him
2. he took a bash right in his face
3. he got a bang on the head
6. find fault with; express criticism of; point out real or perceived flaws
1. The paper criticized the new movie
2. Don't knock the food--it's free
7. deliver a sharp blow or push
1. He knocked the glass clear across the room
8. knock against with force or violence
1. My car bumped into the tree
9. rap with the knuckles
1. knock on the door
10. sound like a car engine that is firing too early
1. the car pinged when I put in low-octane gasoline
2. The car pinked when the ignition was too far retarded
11. make light, repeated taps on a surface
1. he was tapping his fingers on the table impatiently