"Fly" as adjective isn't much used in US,* but in Great Britain it means "mischievous-with-a-touch-of-meanness," yes? (used by Josephine Tey in Brat Farrar to describe Timber, a horse whose only flaw is an inclination to brush off his riders by veering suddenly under a nearby tree and smashing them against a low branch: "He's a fly one, that." )

*I count the fly in "fly paper" as a noun, paper for flies, not an adjective.