Green’s Dictionary of Slang

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

1942		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!
1981		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.
1990		J. Ellroy ‘Gravy Train’ in Pronzini & Adrian Hard-Boiled (1995) 490: ‘Legit?’ ‘If I’m lyin’, I’m flyin’.’.
2002		‘Touré’ Portable Promised Land (ms.) 38: If I’m lyin I’m flyin. And I ain’t seen a feather all day.

(W.I.) gossip, slander.

1950

Bennett, Clarke & Wilson Anancy Stories and Dialect Verse 100: lie an story – gossip.

to lie skilfully and continually.

2001

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.

(Aus.) to tell barefaced lies.

1910		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.
1923		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’.
1941		Baker Popular Dict. Aus. Sl.

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

1876

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].