sly

(as) sly as a fox

Exceptionally clever, cunning, or shrewd, especially in devious or underhanded ways. He thought he was being sly as a fox in the way he had been skimming money from the cash registers, but management caught up with him eventually. The novel centers around the cat-and-mouse chase between a police detective and a thief who is as sly as a fox. She may come off as nothing but smiles and sunshine, but trust me—she's sly as a fox!
See also: fox, sly

a sly dog

Someone who, through cunning, devious, or dishonest means, achieves a fortuitous outcome. What a sly dog, he managed to get a copy of the test so he could memorize the answers ahead of time!
See also: dog, sly

on the sly

Secretly; in a clandestine or furtive manner. Even though he said he'd quit smoking, he still kept up the habit on the sly. The senator had been taking bribes from lobbyists on the sly for years before he was caught.
See also: on, sly
Farlex Dictionary of Idioms.

on the sly

Fig. secretly and deceptively. She was stealing little bits of money on the sly. Martin was having an affair with the maid on the sly.
See also: on, sly

*sly as a fox

 and *cunning as a fox
Cliché smart and clever. (*Also: as ~.) My nephew is as sly as a fox. You have to be cunning as a fox to outwit me.
See also: fox, sly
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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]
See also: on, sly
The American Heritage® Dictionary of Idioms by Christine Ammer.

do something on the ˈsly

do something secretly: She didn’t seem to have much appetite for dinner. I wonder if she’s been eating chocolates on the sly? OPPOSITE: (out) in(to) the open
See also: on, sly, something
Farlex Partner Idioms Dictionary

on the sly

mod. secretly and deceptively. She was stealing little bits of money on the sly.
See also: on, sly

sly

mod. excellent; cool. That is really a sly jacket you got there.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

on the sly

In a way intended to escape notice: took extra payments on the sly.
See also: on, sly
American Heritage® Dictionary of the English Language, Fifth Edition.

on the sly

Secretly, furtively. This early-nineteenth-century term appeared in a letter of John Keats’s (1818): “It might have been a good joke to pour on the sly bottle after bottle into a washing tub.” It remains current.
See also: on, sly
The Dictionary of Clichés by Christine Ammer
See also:
  • (as) sly as a fox
  • sly as a fox
  • (as) wily as a fox
  • (as) crafty as a fox
  • fox
  • crafty
  • cunning
  • (as) cunning as a fox
  • every trick in the book
  • a sly dog
References in periodicals archive
Composed with a lot of passion and a throwback to those days, as the horn lines are purely simply from the cookhouse of music and as has always been the lyrical direction of music from its origins, it is filled with words of wisdom, as Sly tells the youth to stick to the truth, work hard and 'take am easy - oh' as he sings in his favourite tenor to the admiration of those who love the authentic music of the West African coast.
My left hand clutching Sly's arm as though my life depended on it.
He sentenced Sly to four months due to his guilty pleas.
Sly Fox allowed me to experience the entire systems engineering life cycle and gave me an opportunity to network and work with technical experts across several defense agencies.
Sly, who earned PS50,000 for writing and starring in the film, tried sparring with real boxers to find someone to play his opponent Apollo Creed.
Sly, who won the 1,000 Guineas with Speciosa in 2006, added: "It's been a terrible time with all the heavy ground and everything, and now this.
"If I were to describe the relationship between Clare Sly and Derrick Vassell as difficult, that would be an understatement," Mr Balmain said.
Mr Sly was found in his flat in Westcliff-on-Sea, Essex.
"It is with great sorrow that we must say goodbye to Tony Sly of No Use For A Name," said the label's website.
The woman, named locally as Clare Sly, and a man, believed to be her partner, were shot dead at the house in Laneside Gardens, Walsall, in the early hours of yesterday.
Trinity Mirror, the parent company of the Evening Gazette, has announced that Sly Bailey is to step down as Chief Executive.
Sly Bailey said yesterday: "For the past 10 years I have had the privilege of being CEO of Trinity Mirror Plc, a fascinating and all consuming role.
Justin Paul Sly, 20, of no fixed address, is accused of unlawfully and maliciously inflicting grievous bodily harm without intent on Christopher Davies in Llanrwst on March 27 this year.
A major manufacturer of masonry products has successfully employed a new, specially adapted pulse jet dust collector from Sly, Inc.
Sly Inca Newly-incorporated fashion brand from Sylvester