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sly

[slahy]
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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.
Idioms
  1. 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 formssly·ly, adverbsly·ness, nounun·sly, adjective, un·sly·er, un·sly·est.un·sly·ly, adverbun·sly·ness, noun

Synonyms

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

Antonyms

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

sly

adjective slyer, slyest, slier or sliest
  1. crafty; artfula sly dodge
  2. insidious; furtivea sly manner
  3. playfully mischievous; roguishsly humour
noun
  1. on the sly in a secretive manner
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 on the sly

sly

adj.

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

Idioms and Phrases with on the sly

on the sly

Furtively, secretly, as in She's always eating cookies on the sly. The adjective sly, which means “cunning” or “crafty,” is here used as a noun. [c. 1800]

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