c.1200, from O.N.
sloegr "cunning, crafty, sly," from P.Gmc.
*slogis (cf. Low Ger.
slu "cunning, sly"), probably from base
*slog- "hit" (see
slay), with an original notion of "able to hit." Cf. Ger.
verschlagen "cunning, crafty, sly,"
schlagfertig "quick-witted," lit. "ready to strike," from
schlagen "to strike." A non-pejorative use of the word lingered in northern Eng. 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."
[slahy] 
adjective, sly⋅er or sli⋅er, sly⋅est or sli⋅est, noun 