Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Meaning "deprecate, put down" is from 1892. Related: Knocked; knocking. Knock-kneed first attested 1774. Knock-down, drag-out is from 1827. Command knock it off "stop it" is first recorded 1880, perhaps from auctioneer's term for "dispose of quickly:"
At the commencement of the sales, he gave every one that wanted to purchase a paper containing a description of the lands that were to be sold; and, as the sales were cried, he called over the numbers and described the land; and when it got up to one dollar and a quarter an acre, if no body bid, after it was cried two or three times, he would say, knock it off, knock it off. [U.S. Senate record, 1834]
mid-14c., from knock (v.). As an engine noise, from 1899.
: It wasn't a disinterested comment—it was a knock/ The knock on Fernandez is he can't field
"Though Orientals are very jealous of their privacy, they never knock when about to enter your room, but walk in without warning or ceremony. It is nearly impossible to teach an Arab servant to knock at your door. They give warning at the outer gate either by calling or knocking. To stand and call is a very common and respectful mode. Thus Moses commanded the holder of a pledge to stand without and call to the owner to come forth (Deut. 24:10). This was to avoid the violent intrusion of cruel creditors. Peter stood knocking at the outer door (Acts 12:13, 16), and the three men sent to Joppa by Cornelius made inquiry and 'stood before the gate' (10:17, 18). The idea is that the guard over your privacy is to be placed at the entrance." Knocking is used as a sign of importunity (Matt. 7:7, 8; Luke 13:25), and of the coming of Christ (Luke 12:36; Rev. 3:20).