Definitions of knock:

• noun:   the act of hitting vigorously
• noun:   negative criticism
• noun:   a bad experience

  Example: "The school of hard knocks"

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

  Example: "The knocking grew louder"

• noun:   a vigorous blow

  Example: "The sudden knock floored him"

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

  Example: "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

  Example: "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)