"aptly, dexterously, in a skillful manner," mid-15c., from deft + -ly (2). An earlier adverb was daftelike "mannerly, properly, modestly, humbly" (c. 1200).
Entries linking to deftly
deft adj.
"apt or dexterous, subtly clever or skillful," mid-15c., from Old English gedæfte, which meant "mild, gentle, simple, meek," but which splintered into different forms and senses in Middle English, yielding this word and also daft (q.v.). In Middle English it also could mean "well-mannered, gentle, modest, mild," and "dull, uncouth, boorish." Cognate with Gothic gadaban "to be fit," Old Norse dafna "to grow strong," Dutch deftig "important, relevant," from Proto-Germanic *dab-, which has no certain IE etymology and is perhaps a substratum word. Related: Deftness.
-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.