fly
»I did help a chap to sell papers once: he said I was fly at it.» (Tommy And Co. 36. 22.)
A London brat of the working class.
»I don’t take ’er on while I’m myself. I’m too jolly fly.» (Novel Notes 212. 20.)
Uneducated young man.
= knowing, cute.
(Cf. Dickens, Bleak House: »Do what I want, and I will pay you well»—»I’m fly».)
In the 16th and 17th centuries it was held that familiar spirits, in the guise of flies, fleas, etc., attended on witches, who for a price professed to dispose of the Power for evil thus imparted. Thence a fly meant a familiar (spiritus familiaris). That is, I presume, the origin of the above expression.