1550s, "secretly," from close (adj.) + -ly (2). From 1560s as "compactly," 1590s as "so as to enclose;" 1630s as "nearly."
Entries linking to closely
close adj.
(klōs), late 14c., "strictly confined," also "secret," in part a past-participle adjective from close (v.), in part from Old French clos "confined; concealed, secret; taciturn" (12c.), from Latin clausus "close, reserved," past-participle adjective from claudere "stop up, fasten, shut" (see close (v.)). The main sense shifted to "near" (late 15c.) from the verbal sense of "close the gap or opening between two things." Related: Closely.
In English, the meaning "narrowly confined, pent up" is from late 14c. The meaning "near" in a figurative sense, of persons, is from 1560s. The sense of "full of attention to detail" is from 1660s. The sense of "stingy, penurious" is from 1650s. Of races or other contests, by 1855.
Close call "narrow escape" is from 1866, in a quotation in an anecdote from 1863, possibly a term from the American Civil War; close shave in the figurative sense is 1820, American English. Close range (n.) "a short distance" is from 1814. Close-minded is attested from 1818. Close-fisted "penurious, miserly" is from c. 1600, on the notion of "keeping the hands tightly shut."
-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.