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."
mod. excellent; cool. : That is really a sly jacket you got there.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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