Green’s Dictionary of Slang

jawbreaker n.

also jawcracker, jaw-twister

1. [early19C+] a word that the speaker considers so long or complex that its pronunciation threatens to be harmful; thus jawbreaking, incomprehensible, unpronounceable; jawcrackery, the use of such terms.

2. [I] [early 19C] (Aus.) one who writes in a heightened manner.

3. attrib. use of sense 1.

4. [1920s+] (US, also jawbone breaker, jawbone doctor, jawbuster, jaw puller) a dentist.