One of a group of giants with only one eye, and that in the centre of their forehead, whose business it was to forge iron for Vulcan. They were probably Pelasgians, who worked in quarries, and attached a lantern to their forehead to give them light underground. The lantern was their one eye as big as the full moon. (Greek, “circular-eye.”) (See Arimaspians)
Roused with the sound, the mighty family Of one-eyed brothers hasten to the shore, And gather round the bellowing Polypheme.
Addison: Milton Imitated.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Cyclops from Infoplease: