Noun – Knock
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