Meaning

noun a persistently annoying person

noun any of various large flies that annoy livestock

originFrom gad (“sharp point, spike; (dialectal) sharp-pointed rod for driving cattle, horses, etc., goad”) + fly, in the sense of a fly which irritates cattle, etc., by biting them, similar to the prodding of a goad. Gad is derived from Middle English gad, gadde (“metal spike with a sharp point; stick with a sharp point for driving animals, goad; metal bar or rod, ingot; (by extension) lump of…

Etymology from Wiktionary, CC BY-SA 3.0