c. 1200, sleighli, "cautiously, carefully;" c. 1300, "secretly, privately," also "treacherously, stealthily;" late 14c., "wisely, prudently, cleverly;" from sly (adj.) + -ly (2).
Entries linking to slyly
sly adj.
late 12c., sleigh, "skillful, clever, dexterous, wise, prudent," from Old Norse sloegr "cunning, crafty, sly," from Proto-Germanic *slogis (source also of Low German slu "cunning, sly," German schlau), probably from *slak- "to strike, hit" (see slay (v.)), with an original notion of "able to hit." Compare German verschlagen "cunning, crafty, sly," schlagfertig "quick-witted," literally "strike-ready," from schlagen "to strike," but the exact relation is uncertain.
The meaning "insidious, crafty, meanly artful" is from c. 1200. That of "playfully artful, knowing" is from 1764. In Middle English sly words could mean "wise words" or "deceptive language." A non-pejorative use of the word lingered in northern English dialect until 20c. On the sly "in secret" is recorded from 1812. Sly-boots "a seeming Silly, but subtil Fellow" is in the 1700 "Dictionary of the Canting Crew."
-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.