1570s, from Modern Latin heptarchia; see hepta- "seven" + -archy "rule." A group of seven kingdoms; especially in English history in reference to Anglo-Saxon times (Kent, Sussex, Wessex, Essex, Northumberland, East Anglia, Mercia). "The correctness and propriety of the designation have been often called in question, but its practical convenience has preserved it in use" [OED].
Old English Angli Saxones (plural), from Latin Anglo-Saxones, in which Anglo- is an adjective, thus literally "English Saxons," as opposed to those of the Continent (now called Old Saxons). Properly in reference to the Saxons of ancient Wessex, Essex, Middlesex, and Sussex.
I am a suthern man, I can not geste 'rum, ram, ruf' by letter. [Chaucer, "Parson's Prologue and Tale"]
After the Norman-French invasion of 1066, the peoples of the island were distinguished as English and French, but after a few generations all were English, and Latin-speaking scribes, who knew and cared little about Germanic history, began to use Anglo-Saxones to refer to the pre-1066 inhabitants and their descendants. When interest in Old English writing revived late 16c., the word was extended to the language we now call Old English.
In the last years of the reign of Elizabeth, Camden revived the use of the old name Anglosaxones, and, probably for the first time, used lingua Anglosaxonica for the language of England before the Norman conquest. He explains that Anglosaxones means the Saxons of England, in contradistinction to those of the continent; and, in his English Remains, he, accordingly, renders it by "English Saxons." Throughout the seventeenth century, and even later, "English Saxon" continued to be the name ordinarily applied by philologists to the language of king Alfred, but, in the eighteenth century, this gave place to "Anglo-Saxon." [Henry Bradley, in "Cambridge History of English Literature," 1907]
It has been used rhetorically for "English" in an ethnological sense from 1832, and revisioned as Angle + Saxon.