Origin and history of aptly

mid-14c., "inclined, disposed;" late 14c., "suited, fitted, adapted, possessing the necessary qualities for the purpose," from Old French ate "fitting, suitable, appropriate" (13c., Modern French apte), or directly from Latin aptus "fit, suited, proper, appropriate," adjectival use of past participle of *apere "to attach, join, tie to." This is reconstructed to be from PIE root *ap- (1) "to grasp, take, reach" (source also of Sanskrit apnoti "he reaches," Latin apisci "to reach after, attain," Hittite epmi "I seize"). The elliptical sense of "becoming, appropriate" is from 1560s.

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.