middle name (n.)
"portion of a personal name between the given name and the surname," 1815, from middle (adj.) + name (n.). As "one's outstanding characteristic," colloquial, from 1911, American English.
According to Mr. H.A. Hamilton, in his "Quarter Sessions from Queen Elizabeth," the practice of giving children two Christian names was unknown in England before the period of the Stuarts, was rarely adopted down to the time of the Revolution, and never became common until after the Hanoverian family was seated on the throne. "In looking through so many volumes of county records," he says, "I have, of course, seen many thousands and tens of thousands of proper names, belonging to men of all ranks and degrees,—to noblemen, justices, jurymen, witnesses, sureties, innkeepers, hawkers, paupers, vagrants, criminals, and others,—and in no single instance, down to the end of the reign of Anne, have I noticed any person bearing more than one Christian name ...." [Walsh]