“I know you’re lying. You’re talking funny.”
Have you noticed how people often have a different way of speaking when they’re… economizing with the truth? They may use words they wouldn’t otherwise, what a linguist might call “outside of register” – such as stiff, formal ones (altercation, conflagration) or forced-casual ones (folks, heck, ain’t), or an uncomfortable mix of both? They may use a lot of evasive turns of phrase, including we rather than I and agentless passives (“It was done”)? (Don’t take this as hating on passives generally. They’re perfectly useful in some contexts.)
Linguists have terms for different varieties of language. A subset of a language spoken by people from a particular time, place, and social set is a dialect, for instance, and the variety an individual cobbles together that’s distinctive from other speakers of the same dialect is an idiolect. Well, idiolects have their subsets too. The kinds of murmuring inane things you say to a lover or someone you want to be a lover form your erotolect, for instance, and the vocabulary and turns of phrase you slide into when you’re lying are your pseustolect.
Now, if you know Classical Greek, you may be looking a bit squinty-eyed at this. The word for lie, verb, is ψεύδεσθαι pseudesthai, which gave us the root ψευδο- pseudo-. So you might expect pseudolect. But the problem is that pseudo- is widely used and a pseudolect is a fake variety of a language, not a variety of a language used when faking. Which would be why, instead, the related word ψεύστης pseustés ‘liar’ was called in to make this word.
A fun thing about Greek is that it hasn’t changed beyond recognition over the centuries, but it has modified a little; the modern version of ψεύστης is ψεύτης, which has just dropped the sigma (σ, i.e., s), but also in Modern Greek the upsilon (u) is said as [f] before a voiceless consonant. Which would make our word pseftolect, which would be kind of appropriate because it’s misleading – not just because that’s not really an f in the original but because a word it might seem to be that does have an f (or, actually, phi ϕ) there is ψηϕο- psepho-, which relates to voting (as in psephology). But who would want to think that someone might lie, might switch into their lying speech, just for the sake of getting votes? Gosh, hecky-doo, that’s just too rough to conceptualize.
Oh, and shucks, sorry we have to deal with this, but actually this word isn’t an ancient lexeme, though it may pretend to be one. It’s a new old word. (So is erotolect, and the definition of pseudolect is by no means formally established, but you know perfectly well what it would mean if it were because of what we use pseudo to mean.) But it’s no lie, because now that I’ve given it that meaning, it’s the truth. So to speak.