furtively (adv.)

16c., from French furtif (16c.), from Latin furtivus "stolen," hence also "hidden, secret," from furtum "theft, robbery; a stolen thing," from fur (genitive furis) "a thief, extortioner," also a general term of abuse, "rascal, rogue," probably from PIE *bhor-, from root *bher- (1) "to carry; to bear children." Related: Furtiveness.

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.