"the people of England; the speech of England," noun use of Old English adjective Englisc (contrasted to Denisc, Frencisce, etc.), "of or pertaining to the Angles," from Engle (plural) "the Angles," the name of one of the Germanic groups that overran the island 5c., supposedly so-called because Angul, the land they inhabited on the Jutland coast, was shaped like a fish hook (see angle (n.)). The use of the word in Middle English was reinforced by Anglo-French Engleis. Cognates: Dutch Engelsch, German Englisch, Danish Engelsk, French Anglais (Old French Engelsche), Spanish Inglés, Italian Inglese.
Technically "of the Angles," but Englisc also was used from earliest times without distinction for all the Germanic invaders -- Angles, Saxon, Jutes (Bede's gens Anglorum) -- and applied to their group of related languages by Alfred the Great. "The name English for the language is thus older than the name England for the country" [OED]. After 1066, it specifically meant the native population of England (as distinguished from Normans and French occupiers), a distinction which lasted about a generation. But as late as Robert of Gloucester's "Chronicle" (c. 1300) it still could retain a sense of "Anglian" and be distinguished from "Saxon" ("Þe englisse in þe norþ half, þe saxons bi souþe").
... when Scots & others are likely to be within earshot, Britain & British should be inserted as tokens, but no more, of what is really meant [Fowler]
In pronunciation, "En-" has become "In-," perhaps through the frequency of -ing- words and the relative rarity of -e- before -ng- in the modern language. A form Inglis is attested from 14c. and persisted in Scotland and northern England, but the older spelling has stood fast.
Meaning "English language or literature as a subject at school" is from 1889. Old English meaning the Anglo-Saxon language before the Conquest is attested from c. 1200 in an account of the native (as opposed to Latin) month names.