A sound made by a sharp blow or series of blows, especially on a door to attract attention. There was a loud knock at the door, startling everyone in the room.
Losing his job was a real knock to his confidence.
A critical remark or comment. Despite the knocks from critics, the movie was a box office success.
A blow or impact, especially one causing damage or injury. The car had a few knocks and scratches after the minor accident.
An irregular rattling sound in an internal-combustion engine caused by premature ignition of the fuel–air mixture. The mechanic heard a knock in the engine and recommended a tune-up.