[hildea]

I've only skimmed this article: Georgette Heyer and the language of the historical novel, but the conclusion says:

While, much in keeping with her traditional view of history, Heyer avoids lexical and grammatical anachronism, usage is manipulated far beyond the period's own conventions, in order to facilitate the idiom's effectiveness for a modern readership.

So, each word and expression is probably authentic, but often used in ways that are inauthentic? That makes sense, her dialogue is extremely heavy with slang at times.

Originally Posted by Catlady

What are examples of the made-up/not-made-up Regency slang in Heyer?

A couple examples (no idea how made-up or not-made-up this is):

Originally Posted by Mr Chawleigh, a rich man with working class or lower middle class background, from "A Civil Contract"

‘H’m! Didn’t tell you what my business with you is? I thought he would – though he did say he would leave me to lay to you my own way. Damme if he’s not a knowing one! Guessed I’d want more than his testimony before I’d come up to the chalk.’
He nodded, and cast another penetrating stare at Adam.
‘If he had told you what I am he’d have told you that I’m mighty well up in the stirrups. I’m one as likes round dealing – which isn’t to say I won’t get a point the better of a man in a matter of trading, mark you! But there’s no one can say he was clerked by Jonathan Chawleigh! I run no rigs, my lord, because it ain’t my nature, and, what’s more, a good name’s worth a hundred Dutch bargains! I’ve got that all right and regular, and as for my credit, that’s good wherever there’s trading done. You’ll be wanting to know how I made my blunt – for I didn’t come into the world hosed and shod!’

Originally Posted by the Most Honourable the Marquis of Alverstoke, from "Frederica"

'Oh, don’t fly off the hooks again – you can have no notion how bracket-faced you look when you get into one of your pelters! Console yourself with my assurance that if Buxted had left you purse-pinched I should have felt myself obliged to let you hang on my sleeve.’
He looked mockingly down at her.
‘Yes, I know you’re about to tell me that you haven’t sixpence to scratch with, but the plain truth is that you are very well to do in the world, my dear Louisa, but the most unconscionable pinch-penny of my acquaintance! Now, don’t nauseate me by prating of affection! You’ve no more for me than I have for you.’