mid-14c., from whole (adj.) + -ly (2), or a modification of unrecorded Old English *hallice.
Entries linking to wholly
whole adj.
Old English hal "entire, whole; unhurt, uninjured, safe; healthy, sound; genuine, straightforward," from Proto-Germanic *haila- "undamaged" (source also of Old Saxon hel, Old Norse heill, Old Frisian hal, Middle Dutch hiel, Dutch heel, Old High German, German heil "salvation, welfare"), from PIE *kailo- "whole, uninjured, of good omen" (source also of Old Church Slavonic celu "whole, complete;" see health).
The spelling with wh- developed early 15c. The sense in whole number is from early 14c. Whole milk is from 1782. On the whole "considering all facts or circumstances" is from 1690s. For phrase whole hog, see hog (n.).
-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.