on the sly

O

on the sly (idiom)
/ɑːn ðə ˈslaɪ/

Meanings

• Secretly; without letting others notice.
• In a sneaky or hidden way.
• Quietly, often to avoid disapproval.

Synonyms: secretly; stealthily; covertly; furtively; discreetly; sneakily.

Example Sentences

1. Emma checked her messages on the sly during the meeting so that no one would see her using the phone.
2. They continued to meet on the sly because their families did not approve of the friendship.
3. The boy took an extra piece of cake on the sly while everyone was focused on the conversation.

Etymology and Origin

The idiom “on the sly” derives directly from the adjective “sly,” which entered English in the late twelfth century from Old Norse “slœgr,” connoting cunning, crafty, or cleverly skillful. This root traces to a Proto-Germanic source linked to the notion of striking or hitting with precision, originally implying dexterity or agility rather than deceit. Over time the word acquired connotations of stealth and artfulness, though non-pejorative senses of prudent cleverness persisted in certain regional dialects into the modern era.

Development of the Idiom

By the late eighteenth century the construction “on the sly” (or its close variant “upon the sly”) had coalesced as an adverbial expression denoting actions performed secretly, covertly, or without public knowledge. The preposition “on” here functions in its established English role of indicating manner, parallel to formations such as “on the quiet” or “on the cheap,” framing the stealthy quality inherent in “sly.” The idiom thus encapsulated the idea of conducting business or indulging in activities under the rose—privately and often in deliberate concealment.

Geographical Origin

The idiom first emerged within British English, specifically within the cant and vulgar slang of England during the late eighteenth century. It arose in the context of underworld and colloquial speech circulating among London’s lower classes and itinerant populations before spreading more widely through literary and everyday usage across the English-speaking world.

Earliest Printed Evidence

The earliest printed record of the phrase appears in Francis Grose’s A Classical Dictionary of the Vulgar Tongue, first published in 1785. There the entry states that “transacting business privately is frequently said to be done upon the sly,” illustrating its established place in contemporary slang as a descriptor of clandestine dealings. Subsequent appearances, such as in James Hardy Vaux’s 1819 Vocabulary of the Flash Language and John Keats’s private letter of December 1818, reflect the idiom’s continued currency in both cant literature and personal correspondence without altering its core meaning.

Historical Evolution and Persistence

Throughout the nineteenth century the expression gained broader literary acceptance while retaining its association with furtive or illicit behavior, appearing in contexts ranging from social satire to personal anecdote. Its endurance into modern English stems from the enduring utility of a concise formulation for secrecy, free of any competing etymological narratives or alternative origins beyond its transparent derivation from established linguistic patterns.

Variants

• on the quiet
• in secret
• behind someone’s back
• under the radar

Secret