I just learned a fine old expression which Brewer explains efficiently: “To break Priscian’s head (in Latin, Diminuĕre Priscia’ni cap’ut). To violate the rules of grammar. Priscian was a great grammarian of the fifth century, whose name is almost synonymous with grammar.” The locus classicus is Samuel Butler’s long poem Hudibras, in which Quakers are said to “hold no sin so deeply red,/ As that of breaking Priscian’s head” (sc. by using the plural pronoun you in the singular, hence their thee-ing everyone). You can see it in action in the Edinburgh Dramatic Review (Feb. 15, 1825, No. LXXV, Vol. II, p. 298):
People speak of breaking Priscian’s head; but were Priscian in life, we strongly suspect he would break our author’s head for certain glaring solecisms in style,—such as, “as my strongest wishes could expect;” “chaste virginity,” (a phleonism;) “my most sanguine expectations could expect;” “a place vacant;” pro vacated, by some one or other.
I discovered it via Google Books, which brought to my attention J. Y. T. Greig‘s 1929 book Breaking Priscian’s Head: Or, English as She Will Be Spoke and Wrote, which Greig apparently wrote in reaction to an essay by Basil de Sélincourt:
I am not a typical Englishman, but a Scotsman born abroad, and in all the ninety-odd pages of Mr de Sélincourt’s essay on the future of the English language I can find scarcely a paragraph to agree with. Inevitable, that. To nine Scotsmen, ten Americans, and eleven Irishmen, his essay breathes just that spirit of Englishry — rather insular, but oh how gentlemanly ! — which has always infuriated them.
One of his points is that the English should not be so worried about encroaching Americanisms, and in a lively passage on page 83 he says “Certain modern American slang terms are so appropriate and necessary that only incorrigible purists will deny them entrance into good English,” listing, among others, “Bellhop (a page in a hotel, a far better word than page, if only because its use would reduce the number of homophones),” “Blurb (an indispensable word that I am glad to see coming into general use,” “Get one’s goat,” “Junk (which combines into four letters the notions of rubbish and odds-and-ends),” “Movies (a great improvement on cinema),” and “Rubberneck (one of the best words ever coined),” admitting that there are unfortunate terms as well (“the use of mortician for undertaker is ridiculed by the Americans themselves”) but concluding “to cry out for a barrier against all Americanisms as such — that is sheer imbecility.” He then goes on to canvas other sources for “the freshening and replenishment of our language”: “I am not overlooking sources in the British Isles. We have the local dialects, for example, and in particular the virile local dialects of the North, which have been quite absurdly neglected by Southern Englishmen for more than two centuries.” I like his attitude.