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Synonyms

sly

American  
[slahy] / slaɪ /

adjective

comparative

slyer, slier,

superlative

slyest, sliest

  1. cunning or wily.

    sly as a fox.
    Synonyms:
    astute, shrewd, crafty, foxy, subtle, artful
    Antonyms:
    obvious, direct

  2. stealthy, insidious, or secret.

    Synonyms:
    clandestine, underhand, furtive, surreptitious

  3. playfully artful, mischievous, or roguish.

    sly humor.

idioms

  1. on the sly, secretly;

    a tryst on the sly.
sly British  
/ slaɪ /

adjective

  1. crafty; artful

    a sly dodge

  2. insidious; furtive

    a sly manner

  3. playfully mischievous; roguish

    sly humour
"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

noun

  1. in a secretive manner

"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
sly More Idioms  

  1. see on the sly.

Other Word Forms

  • slyly adverb
  • slyness noun
  • unsly adjective
  • unslyly adverb
  • unslyness noun

Etymology

Origin of sly

First recorded in 1150–1200; Middle English sly, sley , from Old Norse slœgr “sly, cunning” (originally “able to strike, able to slay”); slay ( def. )

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

She has a sly way of imparting lessons, often with playfulness.

From Los Angeles Times • Jan. 14, 2026

"Nobody had agreed to it at all. Really sly," fellow chorister Kevin Doughty says in Con Jones: World's Best Conman.

From BBC • Dec. 31, 2025

Here, it is a man whose labor disappears behind a woman’s byline, a sly inversion of the far more familiar historical pattern.

From Salon • Dec. 25, 2025

The math-proficiency analysis suggests it may be doing so on the sly.

From The Wall Street Journal • Nov. 30, 2025

Maddie was watching us with a sly little look on her face, so I cleared my throat and immediately put three feet between me and Sahil.

From "From Twinkle, with Love" by Sandhya Menon

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

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