bellows (n.)

"instrument for producing a current of air," especially for a fire, c. 1200, belwes, literally "bags," plural of belu, belw, northern form of beli, from late Old English belg "pair of bellows; bag, purse, leathern bottle," from PIE *bhelgh- "to swell," extended form of root *bhel- (2) "to blow, swell."

Essentially the same word as belly (n.) and retaining its original sense. It is attested earlier in the specific term blæstbælg, literally "blowing bag," and the modern word is perhaps a reduction of this (compare Old Norse blastrbelgr, German Blasebalg). Used exclusively in plural since 15c., probably due to the two handles or halves.