To break Priscian's head (in Latin, “Diminuere Prisciani caput”). To violate the rules of grammar. Priscian was a great grammarian of the fifth century, whose name is almost synonymous with grammar.
“Priscian's head is often bruised without remorse.” —P. Thompson.
And held no sin so deeply red As that of breaking Priscian's head.
Priscillanists Followers of Priscillian, a Spaniard; an heretical sect which sprang up in Spain in the fourth century. They were a branch of the Manichæans.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894