late 14c., "fit, suitable, proper; affording accommodation; opportune, favorable," from Latin convenientem (nominative conveniens), present participle of convenire "to come together, meet together, assemble; unite, join, combine; agree with, accord; be suitable or proper (to)," from assimilated form of com "with, together" (see con-) + venire "to come" (from a suffixed form of PIE root *gwa- "to go, come").
Meaning "personally suitable to ease of action or performance" is from late 15c. Sense of "at hand, easily accessible" (1849) is marked in OED as "Ireland and U.S."
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.