to knock’ = to hit something or someone hard, e.g. hitting on the door with the hard part of one’s hand (knuckles) in order to call someone; to hit on the head or face, as in boxing

Note 1: There are several expressions with ‘knocker’ used in combination with other words to refer to people and things, most of them slang usually related to sex or sexual activities, like ‘knee knocker’, ‘hob knocker’, ‘turd knocker’, etc. However, there some some expressions with knocker that are fairly normal in usage, like ‘fire knocker’ for a device that puts out fires; the ‘door knocker’ facial hair style (beard) for a ‘goatee’ like beard; a ‘tyre/tire knocker’ for a stick-like device to check the air pressure in vehicle tyres/tires; a ‘head knocker/headknocker’ is a kind of toy that has a movable head, also called ‘bobblehead’; a ‘nigger knocker’ for a stick, or some other stick-like instrument, such as a bat or pipe, used by police to hit criminals; a ‘suggarmaker’s bottom knocker’ is a craftsman of low skill in pottery making; (in football) a ‘snot knocker’ or ‘slobber knocker’ is very powerful hit by an opponent player; a ‘headknocker’ is a very powerful boxer or martial artist; and so on. And ‘knockers‘, always in plural form, is used to refer to a woman’s breast.
Note 2: (less commonly)A ‘knocker’, in Yiddish, is used to refer to ‘a person who thinks great of him/herself and always boasts him/herself’; and ‘knocker-up’ or ‘knocker-upper’ was a person whose job was to wake up people who had to go to work early morning, before the time of alarm clocks, in industrial towns and cities in England and else where. A ‘pork knocker’, on the other hand, is a miner who searches for gold and gems in the interior parts of Guyana, a small country in South America.
Note 3: In computer software, a ‘knocker’ is a program (?). For an article on knocker in computer language, please click here.
Note 4: ‘Knocker’ or ‘Knockers‘, always with a capital ‘K’, is often found in the names of people, e.g. George Knocker, Norman Alan Knocker, Lillian Knocker, Johanna Knockers, etc.

‘Knocker’ is also used for a metal device fixed on a door, usually in the shape of the head of a lion or a ring,  that makes a loud noise when hit against the door so that the caller does not need to use his/her hand to knock on the door. For an article that explains how to fix a door kncoker on a door, please click here.