Full Definition of GADFLY. 1. : any of various flies (as a horsefly, botfly, or warble fly) that bite or annoy livestock. 2. : a person who stimulates or annoys especially by persistent criticism.See gadfly defined for English-language learners. See gadfly defined for kids.ull Definition of GADFLY. 1. : any of various flies (as a horsefly, botfly, or warble fly) that bite or annoy livestock. 2. : a person who stimulates or annoys especially by persistent criticism. See gadfly defined for English-language learners. See gadfly defined for kids.