"in an ominous manner," 1590s, from ominous + -ly (2). In earliest use, "with good omen, auspiciously," but this sense has been obsolete since late 17c.; the main modern meaning "with evil omen" is attested from 1640s (Milton).
"conveying an omen, significant," 1580s, from Latin ominosus "full of foreboding," from omen (genitive ominis) "foreboding" (see omen (n.)). Especially (and now exclusively) "of ill omen, giving indication of coming evil." Related: Ominousness.
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.