adjective, slyer or slier, slyest or sliest.
cunning or wily: sly as a fox.
stealthy, insidious, or secret.
playfully artful, mischievous, or roguish: sly humor.
on the sly, secretly; furtively: a tryst on the sly.

1150–1200; Middle English sly, sley < Old Norse slœgr sly, cunning

slyly, slily, adverb
slyness, noun
unsly, adjective, unslyer or unslier, unslyest or unsliest.
unslyly, unslily, adverb
unslyness, noun

1. artful, subtle, foxy, crafty, shrewd, astute. 2. surreptitious, furtive, underhand, clandestine.

1. direct, obvious. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
sly (slaɪ)
adj , slyer, slyest, slier, sliest
1.  crafty; artful: a sly dodge
2.  insidious; furtive: a sly manner
3.  playfully mischievous; roguish: sly humour
4.  on the sly in a secretive manner
[C12: from Old Norse slǣgr clever, literally: able to strike, from slā to slay]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

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."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Idioms & Phrases


see on the sly.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Simply, they're a clever way to capture sights and sounds on the sly while your
  subject remains none the wiser.
What separated him from a run-of-the-mansion celebrity portraitist is his
  offhand intimacy and sly wit.
Heck, if you want to be sly, duck into the confessional and discuss it in
  private out of view.
You're also pulling a sly trick when you compare safety versus coal plants.
Idioms & Phrases
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