sly

UK: /ˈslaɪ/ US: /slaɪ/ ,(slī)

Inflections of 'sly' (adj):

slyer

adj comparative

slier

adj comparative

slyest

adj superlative

sliest

adj superlative

WordReference Random House Learner's Dictionary of American English © 2026

sly /slaɪ/   adj., sly•er or sli•er, sly•est or sli•est, n. 
adj. 

1. sneaky;
   tricky;
   cunning:
   The sly old fox was able to outsmart us once again.
2. able to avoid being seen or noticed;
   stealthy:
   a sly move.
3. mischievous:
   sly humor.

n., Idiom.

4. on the sly, [uncountable] secretly;
   without others knowing or seeing.

sly•ly, adv.: 

slyly slipped the money from the drawer.

sly•ness, n. [uncountable]

WordReference Random House Unabridged Dictionary of American English © 2026

sly  (slī), adj., sly•er or sli•er, sly•est or sli•est, n. 
adj. 

1. cunning or wily:
   sly as a fox.
2. stealthy, insidious, or secret.
3. playfully artful, mischievous, or roguish:
   sly humor.

n. 

4. on the sly, secretly;
   furtively:
   a tryst on the sly.

• Old Norse slœgr sly, cunning
• Middle English sly, sley 1150–1200

sly′ly, sli′ly, adv. 
sly′ness, n. 
Synonyms

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

Antonyms

• 1. direct, obvious.

Collins Concise English Dictionary © HarperCollins Publishers::

sly / slaɪ/ adjective (slyer, slyest, slier, sliest)

1. crafty; artful: a sly dodge
2. insidious; furtive: a sly manner
3. playfully mischievous; roguish: sly humour

noun

1. on the sly ⇒ in a secretive manner

Etymology: 12th Century: from Old Norse slǣgr clever, literally: able to strike, from slā to slay

ˈslyly, ˈslily adverb

ˈslyness noun