conclusively (adv.)

1610s, "occurring at the end," from French conclusif, from Late Latin conclusivus, from conclus-, past participle stem of Latin concludere "to shut up, enclose," from assimilated form of com "together" (see con-) + -cludere, combining form of claudere "to shut" (see close (v.)). Meaning "definitive, decisive, convincing, being so forcible as not to admit of contradiction" (on the notion of "leading to a logical conclusion," and thus putting an end to debate) is from 1640s. Related: Conclusiveness.

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.