knock

A vigorous blow.

“the sudden knock floored him”

A bad experience.

“the school of hard knocks”

The act of hitting vigorously.

The sound of knocking (as on a door or in an engine or bearing).

“the knocking grew louder”

Negative criticism.

Rap with the knuckles.

“knock on the door”

Find fault with; express criticism of; point out real or perceived flaws.

“Don't knock the food--it's free”

Deliver a sharp blow or push.

“He knocked the glass clear across the room”

Knock against with force or violence.

Make light, repeated taps on a surface.

Sound like a car engine that is firing too early.