What it means
In UK and Irish slang, if someone's fly, they're sharp, clued-up, and not easy to catch slipping. They spot a hustle early and might have a cheeky little move of their own too. It can be praise, but there's often a tiny don't-get-too-comfy warning tucked into it. In some places it also means stylish or cool, but the crafty, switched-on sense is the older one.
Usage examples
"He blagged a free room upgrade, swiped the mini shampoos, then pulled the innocent face at reception. Proper fly, that lad. I paid like a mug."
"The new postmaster of the village shop of the Wicklow countryside of Ireland is a fly character of the local community, has identified within the first three months of the appointment which customers of the post office round of the rural route pay the bills on time and which ones leave the cheque uncashed at the kitchen drawer of the family home of the next generation."
"The fly second-hand car dealer of the industrial estate of the Belfast outskirts has the reputation of the entire community of the working-class neighbourhood, sells the used vehicles of the lot at the right margin of profit between the wholesale price of the auction of the wholesale lot and the retail price of the local market of the family customer base."
"Don't let the soft voice fool you, she's fly as anything. She saw the landlord's nonsense coming a mile off."
"Yer man at the market is fly. Smile on his face, hand in your pocket before you've even copped what happened."
Where it comes from
This sense of fly is old British slang, going back to late 18th century cant and underworld talk. Back then it meant sharp, knowing, and hard to fool. It shows up in criminal writing and low-life slang from that era, then kept its feet in British and Irish English as a sly nod to someone clever with a dodgy edge.
