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