Quick definitions from WordNet (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:  rap with the knuckles ("Knock on the door")
▸ verb:  knock against with force or violence
▸ verb:  sound like a car engine that is firing too early
▸ verb:  make light, repeated taps on a surface
▸ name:  A surname (very rare: popularity rank in the U.S.: #56249)

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▸ Also see knock