Mambrino was a fictional Moorish king, celebrated in the romances of chivalry. According to legend, Mambrino possessed a helmet of pure gold that rendered its wearer invulnerable. Possession of the helmet was the ambition of all the paladins of Charlemagne, and it was carried off by first by Gradasso, King of Sericane, and a second time by Rinaldo (Orlando Furioso), who slew Gradasso at Barcelona.
Cervantes, in his novel Don Quixote de la Mancha, tells us of a barber who was caught in the rain, and to protect his hat clapped his brazen basin on his head. Don Quixote insisted that this basin was the enchanted helmet of the Moorish king. Don Quixote wishes to obtain the helmet in order to make himself invulnerable. In the musical Man of La Mancha, an entire song is constructed around the titular character's search for the helmet and his encounter with the barber.
There is a reference in Fermor's "Mani" to Mambrino with respect to a very large straw hat worn by a Greek man in the 1930s. "[The man] came loping towards us under his giant Mambrino's helmet of straw."