A clown in Love's Labour's Lost (Shakespeare), who apes the court wit of Queen Elizabeth's time, but misapplies and miscalls like Mrs. Malaprop or Master Dogberry.
A large apple, and, metaphorically, a man's head. (See Costermonger.)
“Take him over the costard with the hilts of thy sword.” —Shakespeare: Richard III., i. 4.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894