(Gawain and some of Arthur’s other knights are at the court of the leader of the Romans, Lucius Hibernius)
As Lucius was replying that he had not come there in order to withdraw, but rather that he might govern the country, his nephew Gaius Quintillanus who was present was heard to mutter that the Britons were better at boasting and making threats than they were at proving their courage and prowess on the battle-field. Gawain was immediately incensed at this. He drew his sword from the scabbard which was hanging at his belt, rushed at Gaius and cut off his head.
(the Britons make a run for it as the Romans give chase)
In the meantime Marcellus Mutius was making every effort to avenge Quintillianus. He was already threatening Gawain from the rear, and was on the point of laying hold of him, when Gawain swung around and with the sword which he brandished clove him through helm and head to his chest, bidding him, when he got to hell, to tell Quintillianus, whom Gawain had just cut down in camp, that this was why the Britons were so good at boasting and making threats.
Geoffrey of Monmouth, The History of the Kings of Britain, x.4