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adjective, sly·er, sly·est.
  1. cunning or wily: sly as a fox.
  2. stealthy, insidious, or secret.
  3. playfully artful, mischievous, or roguish: sly humor.
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  1. on the sly, secretly; furtively: a tryst on the sly.
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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 formssly·ly, adverbsly·ness, nounun·sly, adjective, un·sly·er, un·sly·est.un·sly·ly, adverbun·sly·ness, noun


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


1. direct, obvious.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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 or sliest
  1. crafty; artfula sly dodge
  2. insidious; furtivea sly manner
  3. playfully mischievous; roguishsly humour
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  1. on the sly in a secretive manner
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Derived Formsslyly or slily, adverbslyness, 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

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."

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Online Etymology Dictionary, © 2010 Douglas Harper

Idioms and Phrases with sly


see on the sly.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.