Green’s Dictionary of Slang

1. the practice of posing as a fashionable gentleman to swindle the gentry.

1868

‘Six Years in the Prisons of England’ in Temple Bar Mag. Nov. 535: ‘Don’t you think “highflying” would suit me better, although I know little about it?’ ‘O, that’s above your mark; a “highflyer” is a bloke who dresses like a clergyman, or some gentleman. He must be educated, for his game is to know all the nobility and gentry, and visit them with got-up letters [...] for the purpose of getting subscriptions to some scheme.’.

2. (US) immorality, hedonism, extravagance.

1875		J.G. Holland Sevenoaks 292: I have to [...] promise to raise the divil wid her whiniver she gits a fit o’ high flyin’.
1942		Berrey & Van den Bark Amer. Thes. Sl.