early 15c., secretli, "in secret, confidentially, in private, without the knowledge or observation of others," from secret (adj.) + -ly (2). Earlier was secrely (late 14c.), from secre (adj.).
Entries linking to secretly
secret adj.
late 14c., "set or kept apart, hidden, concealed," from French secret, adjective use of noun, from Latin secretum "a secret, a hidden thing" (see secret (n.)).
Secret agent is recorded by 1715; secret service is from 1737, "department of a government concerned with counterfeiting and other political and civil offenses done in secrecy;" secret police is by 1823. Secret weapon is by 1590s.
-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.