Merriam-Webster Logo
  • Dictionary
  • Thesaurus
  • Scrabble
  • Spanish Central
  • Learner's Dictionary


adjective, \ˈslī\

Simple Definition of sly

  • : clever in a dishonest way

  • : showing that you know a secret

Source: Merriam-Webster's Learner's Dictionary

Full Definition of sly

slier or slyer play \ˈslī(-ə)r\ sliest or slyest play \ˈslī-əst\

  1. 1 chiefly dialect a :  wise in practical affairsb :  displaying cleverness :  ingenious

  2. 2a :  clever in concealing one's aims or ends :  furtiveb :  lacking in straightforwardness and candor :  dissembling

  3. 3 :  lightly mischievous :  roguish <a sly jest>

slyly also slily adverb
slyness noun
on the sly
  1. :  in a manner intended to avoid notice

Examples of sly in a sentence

  1. <the movie pairs a sly, dissembling ex-con with an upstanding, straight-arrow cop>

  2. <why, you sly fellow! I had no idea you were planning my birthday party>

Origin of sly

Middle English sleighe, sli, from Old Norse slœgr; akin to Old English slēan to strike — more at slay

First Known Use: 13th century

Synonym Discussion of sly

sly, cunning, crafty, wily, tricky, foxy, artful, slick mean attaining or seeking to attain one's ends by guileful or devious means. sly implies furtiveness, lack of candor, and skill in concealing one's aims and methods <a sly corporate raider>. cunning suggests the inventive use of sometimes limited intelligence in overreaching or circumventing <the cunning fox avoided the trap>. crafty implies cleverness and subtlety of method <a crafty lefthander>. wily implies skill and deception in maneuvering <the wily fugitive escaped the posse>. tricky is more likely to suggest shiftiness and unreliability than skill in deception and maneuvering <a tricky political operative>. foxy implies a shrewd and wary craftiness usually involving devious dealing <a foxy publicity man planting stories>. artful implies indirectness in dealing and often connotes sophistication or cleverness <elicited the information by artful questioning>. slick emphasizes smoothness and guile <slick operators selling time-sharing>.

SLY Defined for Kids


adjective \ˈslī\

Definition of sly for Students

slier or slyersliest or slyest

  1. 1 :  both clever and tricky

  2. 2 :  tending to keep secrets and hide intentions

  3. 3 :  mischievous 2 <There were teachers all over the place, and any kind of fighting or taunting, above a sly pinch … was out. — Lynne Reid Banks, The Indian in the Cupboard>

slyly adverb
slyness noun
on the sly
  1. :  so as not to be seen or caught :  secretly

Seen and Heard

What made you want to look up sly? Please tell us where you read or heard it (including the quote, if possible).


to dishevel or rumple

Get Word of the Day daily email!


Take a 3-minute break and test your skills!


Which of these is a synonym of nonplus?

reduce soothe disapprove perplex
Name That Thing

Test your visual vocabulary with our 10-question challenge!


Test Your Knowledge - and learn some interesting things along the way.