1550s as an adjective, "pertaining to or being in or toward the north; proceeding from the north," from northern + -ly (2) on pattern of easterly, westerly. As an adverb, "toward the north," from 1590s. Related: Northerliness.
"of or pertaining to a region, place, or point nearer the north than some other," Old English norþerna, norðerne "northern, of the north; Northumbrian; Scandinavian," cognate with Old High German nordroni, Old Norse norroenn (see north). With -erne, suffix denoting direction. Related: Northernmost.
Northerner "man from the north of England" is attested from late 13c. as a surname. In the U.S. sense "native or resident of the northern states or territories" it is attested by 1818. Northern lights "aurora borealis" is recorded by that name by 1721 (earlier north-light, 1706).
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.