From Old English 'belg' (bag), from PIE *bhelgh- (to swell)—a bellows is a 'bag' that puffs with air.
A device with an air chamber and handles used to blow a stream of air onto a fire, especially in a forge or furnace.
From Old English 'belg, bælg' (bag, leather sack), from Proto-Germanic *balgiz (bag, skin), from PIE *bhelgh- (to swell). A bellows is etymologically a 'bag' that swells with air. The plural form became standard because the tool has two sides. Key roots: *bhelgh- (Proto-Indo-European: "to swell, to puff up").