Green’s Dictionary of Slang

lie v.2

SE, meaning to tell untruths, in slang uses

In phrases

if I’m lying I’m dying (also if I’m lying I’m flying)

(US black) a phr. implying the speaker’s attestation of absolute honesty and good faith.

[US]Z.N. Hurston ‘Story in Harlem Sl.’ in Novels and Stories (1995) 1002: How can I be broke when I got de best woman in Harlem? [...] If I’m lying, I’m flying!
[US]J. Wambaugh Glitter Dome (1982) 80: ‘I dunno, I dunno, Weasel,’ Sox Wilson whined. ‘If I’m lyin, I’m flyin.’ ‘If you’re lyin, you’re fryin,’ the Weasel corrected him [...] ‘If you’re lyin, you’re dyin,’ Ferret corrected them both.
[US]J. Ellroy ‘Gravy Train’ in Pronzini & Adrian Hard-Boiled (1995) 490: ‘Legit?’ ‘If I’m lyin’, I’m flyin’.’.
[US]‘Touré’ Portable Promised Land (ms.) 38: If I’m lyin I’m flyin. And I ain’t seen a feather all day.
lie like a flatfish (v.) [pun]

to lie skilfully and continually.

ThisisYork.co.uk 24 Jul. 🌐 So Lord Archer of Weston-Super-Mud, the best-selling storyteller, with the ability to lie like a flatfish, has at last been brought to book.
lie like a pig (in shit) (v.)

(Aus.) to tell barefaced lies.

Wagga Wagga Exp. 1 Feb. 2/8: He had asked her to return to him, but witness said this was not until after the issue or the summons. ‘You lie like a pig,’ said the defendant.
[Aus]Morn. Bulletin (Rockhampton, Qld) 8 Feb. 9/8: He was accorded the following responses [...] ‘You are making untrue statements and lie like a pig in a pigsty’.
[Aus]Baker Popular Dict. Aus. Sl.
[Aus]S .J. Baker Aus. Vulgarisms [t/s] 10: lie like a pig in shit: To prevaricate, to be a double-dyed liar.
lie like truth (v.)

to lie in a plausible manner, often used of cheapjacks and other street salesmen.

[UK]C.Hindley Life and Adventures of a Cheap Jack 3: The fraternity... are always supposed, and by common consent allowed, to lie like truth [F&H].