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[slahy] /slaɪ/
adjective, slyer, slyest.
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.
Origin of sly
1150-1200; Middle English sly, sley, from Old Norse slœgr “sly, cunning” (originally “able to strike, able to slay”); see slay
Related forms
slyly, adverb
slyness, noun
unsly, adjective, unslyer, unslyest.
unslyly, 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. 2018.
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Examples from the Web for sly
Contemporary Examples
Historical Examples
  • "Let them sleep in the bed of honor," said the Princess Medea, with a sly smile at Jason.

    Tanglewood Tales Nathaniel Hawthorne
  • The sly, wicked Mimi came slinking to the place where the dragon lay.

  • I know her sly manner of feeling her way with those gloves of hers.

    Little Dorrit Charles Dickens
  • The faces seemed to have but one smile, conscious, sly, a little alarmed.

    The Coryston Family Mrs. Humphry Ward
  • But I had only to open my lips to speak, and away she would run, with a sly smile.

    A Hero of Our Time M. Y. Lermontov
British Dictionary definitions for sly


adjective slyer, slyest, slier, sliest
crafty; artful: a sly dodge
insidious; furtive: a sly manner
playfully mischievous; roguish: sly humour
on the sly, in a secretive manner
Derived Forms
slyly, slily, adverb
slyness, noun
Word Origin
C12: from Old Norse slǣgr clever, literally: able to strike, from slā to slay
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for sly

c.1200, "skillful, clever, dexterous," from Old Norse sloegr "cunning, crafty, sly," from Proto-Germanic *slogis (cf. Low German slu "cunning, sly," German schlau), probably from base *slak- "to strike, hit" (see slay (v.)), with an original notion of "able to hit." Cf. German verschlagen "cunning, crafty, sly," schlagfertig "quick-witted," literally "strike-ready," from schlagen "to strike." 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."

Online Etymology Dictionary, © 2010 Douglas Harper
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Idioms and Phrases with sly


see: on the sly
The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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