"without a name, in an anonymous manner," 1728, from anonymous + -ly (2).
Entries linking to anonymously
anonymous adj.
c. 1600, "without a name;" 1670s, "published under no name, of unknown authorship," from Late Latin anonymus, from Greek anonymos "without a name," from an- "without" (see an- (1)) + onyma, Æolic dialectal form of onoma "name" (from PIE root *no-men- "name").
-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.