The long-standing popularity of John has spawned numerous expressions in English, not all of them likely to please bearers of the name.
Because the name is so common, it has served as a generic appellation for any man, rather like Mac, Jack, and Joe, as in Hey, Mac! Got a light?, Every man jack of them (meaning every single man), and G.I. Joe (any man serving in the military).
In the days of the great English manors (think, Downton Abbey), “John” or “John Thomas” was used to refer to a man of the servant class such as a footman, butler, or waiter. In time, “John Thomas” became a euphemism for penis. Another word for the same male body part is “Johnson.”
An anglicized version of French gendarme gave “johndarm” or “john,” a slang word for a policeman.
“John Doe” originated in English law as a fictitious name to describe one of the people in person a certain type of litigation. The name has come to be used to refer to an ordinary or typical citizen. A 1941 Frank Capra movie starring Gary Cooper and Barbara Stanwyck, Meet John Doe, focuses on the suffering of homeless and unemployed U.S. citizens.
Viewers of police dramas know that “John Doe” is often used to refer to a corpse whose identity is unknown. A female victim is called a “Jane Doe.”
The word john to refer to a W.C. or toilet may derive from the name of its inventor, Sir John Harington/Harrington (1561-1612), a member of Queen Elizabeth I’s court.
The use of john to refer to a prostitute’s client may have something to do with one of the meanings of “John Thomas.”
The word john also occurs in a few noun compounds.
A demijohn is a large bottle usually encased in wicker, like a bottle of Chianti, only much bigger. The wicker casing has one or two handles to make the bottle easier to carry.
A John boat is a small, flat-bottomed boat used on inland waterways in the U.S. It is also spelled jon boat.
A johnny cake in the U.S. is a cake made of cornmeal and toasted before a fire. In Australia, a johnny cake is made of wheat meal and baked on the ashes or fried in a pan.