noun

• (figuratively, informal) A criticism.
  "Since forming in 2007 Mumford & Sons have hard-toured their way to a vast market for throaty folk that's strong on banjo and bass drum. They have released two enormous albums. But, wow, do they take some knocks back home."
• (figuratively, informal) A blow or setback.
  ""Come on!" cried Mr. Beaver, who was almost dancing with delight. "Come and see! This is a nasty knock for the Witch! It looks as if her power was already crumbling.""
• (automotive, uncountable) Preignition, a type of abnormal combustion occurring in spark ignition engines caused by self-ignition; also, the characteristic knocking sound associated with it.
• (cricket, slang) A batsman's innings.
  "He had a good knock and scored well."

verb

• (intransitive) To rap one's knuckles against something, especially wood.
  "Knock on the door and find out if they’re home."
• (transitive, dated) To strike for admittance; to rap upon, as a door.
  "Master, knock the door hard."
• (transitive, colloquial, originally US) To criticize verbally; to denigrate; to undervalue.
  "Don’t knock it till you’ve tried it."
• (transitive, soccer) To kick a ball towards another player; to pass.