"concisely, with little length," c. 1300, from brief (adj.) + -ly (2). As an introduction to a statement, "in short," it is recorded from 1510s.
Entries linking to briefly
brief adj.
c. 1300, bref, "of short duration;" early 14c., "small with respect to length, short;" from Latin brevis (adj.) "short, low, little, shallow," from PIE *mregh-wi-, from root *mregh-u- "short."
-ly 2
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.