Green’s Dictionary of Slang

of words and speech, hard to pronounce.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: break-teeth words Hard words, difficult to pronounce.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd edn).
1811		Lex. Balatronicum [as cit. 1788].
1814		Last Act in New British Theatre I iii: It was zome break-jaw latins, or Irish, or some other larned languages.
1816		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!
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1788].
1839		North. Liberator (Tyne & Wear) 26 Jan. 3/4: Horrible crack-jaw word that!
1844		G.W. Kendall Narrative of Texan Santa Fe Expedition I 107: The crack-jaw name of some Indian brave.
1853		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.
1866		Wild Boys of London I 20/2: We ain’t a-frightened though they can use crackjaw words.
1872		Chambers’s Misc. No. 152 2: The most jawbreaking polysyllables were cleared in a flying gallop [F&H].
1906		Seattle Republican (WA) 27 Apr. 2/2: The nerve-wracking, jawbreaking names usually given to [railroad] drawing rooms and sleping cars.
1911		J. Lomax Cowboy Songs 155: He astonished all them cowboys with them jawbreaking words.
1920		Eve. World (NY) 3 June 10/1: [headline] Old ills Trotted Out Under Jaw-Breaking Names to Aid John Barleycorn.
1935		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.
1958		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.
a.1986		‘Old Zebra Dun’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 82: He drove the punchers crazy with his durn jawbreakin’ words.