1570s, "predestined," from fatal + -ly (2). Meaning "in a deadly manner" is from 1590s.
Entries linking to fatally
fatal adj.
ldlate 14c., "decreed by fate," also "fraught with fate," from O French fatal (14c.) and directly from Latin fatalis "ordained by fate, decreed, destined; of or belonging to fate or destiny; destructive, deadly," from fatum (see fate (n.)). Original senses are obsolete; the meaning "causing or attended with death" in English is from early 15c. Meaning "concerned with or dealing with destiny" is from mid-15c.
-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.