1797, ancient supernatural people of Ireland (enemies of the Dannans); according to OED perhaps from Old Irish fir, plural of fear "man" + bolg, genitive plural of bolg "bag, belly" (from PIE *bhelgh- "to swell," extended form of root *bhel- (2) "to blow, swell"). Or the second element may be cognate with the Gaulish tribal name Belgae. Related: Firbolgian.
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