I know the meaning of 'lockjaw', which is a kind of disease but I don't think this word from the sentence below doesn't literally means that disease but other thing.
This is some part from a novel 'Baker's Blue-jay yarn' by Mark Twain.
[You may say a cat uses good grammar. Well, a cat does - but you let a cat get excited once; you let a cat get to pulling fur with another cat on a shed, nights, and you'll hear grammar that will give you the lockjaw.]
Lockjaw is the vernacular name for the disease tetanus, much more common in the west before a vaccine was developed in the 1920s. It causes extreme muscle spasm, usually starting with the muscles of the jaw—hence the name.
Twain's narrator employs the term for comic exaggeration, alluding to a common reaction to recurrent annoyances: wincing and clenching the teeth to forestall vehement protest.
According to the Oxford Dictionary, lockjaw can also refer to:
Spasm of the jaw muscles, causing the mouth to remain tightly closed, typically as a symptom of tetanus.
The technical medical term is trismus.
This doesn't mean that it can't apply to other circumstances, as defined by the book Clinical Problem Solving in Dentistry:
The definition of trismus is reduced opening caused by spasm of the muscles of mastication but the term is used loosely for all causes of limited opening. True trismus is usually temporary.
In the novel, I'd say that it refers to the fact that once you let the cat get excited, the grammar you hear will make you unable to open your mouth, which you can corroborate with the next sentence in the novel.
Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use
