Green’s Dictionary of Slang

jaw-breaking adj.

also break-jaw, break-teeth, crack-jaw
[jawbreaker n.]

of words and speech, hard to pronounce.

[UK]Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: break-teeth words Hard words, difficult to pronounce.
[UK]Grose Classical Dict. of the Vulgar Tongue (2nd edn).
[UK]Lex. Balatronicum [as cit. 1788].
[UK]Last Act in New British Theatre I iii: It was zome break-jaw latins, or Irish, or some other larned languages.
[UK]J.H. Lewis Lectures on Art of Writing (1840) 62: Fokes of little heddecation, who don’t know what is the meaning of hard crack-jaw words!
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1788].
[UK]North. Liberator (Tyne & Wear) 26 Jan. 3/4: Horrible crack-jaw word that!
[US]G.W. Kendall Narrative of Texan Santa Fe Expedition I 107: The crack-jaw name of some Indian brave.
[US]T. Haliburton Sam Slick’s Wise Saws I 308: Such words too, [...] not jaw-breakin’ words, such as black gentlemen use to show their knowledge of dictionary, but heart-breakin’ words.
[UK]Wild Boys of London I 20/2: We ain’t a-frightened though they can use crackjaw words.
Chambers’s Misc. No. 152 2: The most jawbreaking polysyllables were cleared in a flying gallop [F&H].
[US]Seattle Republican (WA) 27 Apr. 2/2: The nerve-wracking, jawbreaking names usually given to [railroad] drawing rooms and sleping cars.
[US]J. Lomax Cowboy Songs 155: He astonished all them cowboys with them jawbreaking words.
[US]Eve. World (NY) 3 June 10/1: [headline] Old ills Trotted Out Under Jaw-Breaking Names to Aid John Barleycorn.
[US]J. Conroy World to Win 81: Martha had had the ‘power’ three times in meetings at the church, and each time she had talked [...] using more weird-sounding and jaw-breaking words than even Pastor Epperson had before.
[US]C. Himes Real Cool Killers (1969) 153: The commissioner had come into office by way of a law practice and could handle these jaw-breaking words.
[US] ‘Old Zebra Dun’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 82: He drove the punchers crazy with his durn jawbreakin’ words.