knock / näk/ • v. 1. [intr.] strike a surface noisily to attract attention, esp. when waiting to be let in through a door: I knocked on the kitchen door. ∎  strike or thump together or against something: my knees were knocking and my lips quivering. ∎  (of a motor or other engine) make a regular thumping or rattling noise because of improper ignition. 2. [tr.] collide with (someone or something), giving them a hard blow: he deliberately ran into her, knocking her shoulder | [intr.] he knocked into an elderly man. ∎  [tr.] force to move or fall with a deliberate or accidental blow or collision: he'd knocked over a glass of water. ∎  injure or damage by striking: she knocked her knee painfully on the table. ∎  make (a hole or a dent) in something by striking it forcefully: he suggests we knock a hole through the wall into the broom closet. ∎ inf. talk disparagingly about; criticize.• n. 1. a sudden short sound caused by a blow, esp. on a door to attract attention or gain entry. ∎  a continual thumping or rattling sound made by an engine because of improper ignition. 2. a blow or collision: the casing is tough enough to withstand knocks. ∎  an injury caused by a blow or collision. ∎  a discouraging experience; a setback: the region's industries have taken a severe knock. ∎ inf. a critical comment.PHRASES: knock it off inf. used to tell someone to stop doing something that one finds annoying or foolish.