The nickname given to Gawain after he was knighted by the Emperor of Rome.
Ignorant of his real name during his upbringing, he was first called the Boy with No Name, but was dubbed the Knight of the Surcoat after his fellow knights were surprised to see him wear a tunic over his armor – a custom then unknown in Rome.
Source
De Ortu Waluuanii Nepotis Arturi | Late 13th century