"capable of being done, accomplished or carried out," mid-15c., from Anglo-French faisible, from Old French faisable "possible, that may be done; easy, convenient," from fais-, stem of faire "do, make," from Latin facere "to make, do, perform" (from PIE root *dhe- "to set, put"). Fowler recommends this word only for those "who feel that the use of an ordinary word for an ordinary notion does not do justice to their vocabulary or sufficiently exhibit their cultivation."
-ly 2
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.