Origin and history of distinctly

late 14c., "not identical, not the same," also "clearly perceptible by sense," past-participle adjective from obsolete distincten (c. 1300) "to distinguish one thing from another; make distinct," from Old French distincter, from Latin distinctus, past participle of distinguere "to separate between, keep separate, mark off" (see distinguish). Meaning "plain and intelligible to the mind" is from c. 1600. Related: Distinctness.

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is 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.