Definition of "Knock" :

noun: the act of hitting vigorously

noun: negative criticism

noun: a bad experience

"The school of hard knocks."

noun: the sound of knocking (as on a door or in an engine or bearing)

"The knocking grew louder."

noun: a vigorous blow

"The sudden knock floored him."

verb: find fault with; express criticism of; point out real or perceived flaws

"Don't knock the food--it's free."

verb: deliver a sharp blow or push

"He knocked the glass clear across the room."

verb: knock against with force or violence

verb: rap with the knuckles

"Knock on the door."

verb: sound like a car engine that is firing too early

verb: make light, repeated taps on a surface