In UK and Irish slang, fly means sly, sharp, a bit too clever for your own good. Like someone clocking the scam before you’ve even opened your mouth. It can also mean stylish or cool, which overlaps with American slang, but over here it’s often more about being sneaky-smart than looking fresh. Honestly, it’s a compliment with side-eye.
"He blagged a free upgrade, nicked the mini shampoos, then acted innocent at reception. Proper fly, that lad. Meanwhile I’m paying extra for late checkout like a mug."