late 14c., "pleasantly, enjoyably," from comfortable + -ly (2). Meaning "in a state of comfort" is 1630s.
Entries linking to comfortably
comfortable adj.
mid-14c., "affording mental or spiritual comfort," from Anglo-French and Old French confortable "comforting; pleasant, agreeable," from conforter "to comfort, solace" (see comfort (v.)); also see -able. Meaning "cheering, cheerful" is from c. 1400. Meaning "offering physical comfort" is attested from 1769; that of "in a state of tranquil enjoyment" is from 1770.
-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.