1 a [count] : a hard, sharp hit
He gave him a knock on the head.
1 b [count] : the sound made by a hard, sharp hit
There was a loud knock at the door.
2 [count] informal : an experience that makes you less confident or successful for a period of time : a difficult or painful experience
She took some knocks early in her career.
Most performers have their share of knocks [=setbacks] on their way to stardom.
3 [count] informal : a critical or negative comment
He likes praise but can't stand the knocks.
4 : a loud noise produced by an engine when it is not working properly
We heard a knock in the engine.
a type of fuel that reduces engine knock