gadfly

noun

1. 1

   Any dipterous insect of the family Oestridae, commonly known as botflies.

2. 2

   A horsefly: any of various species of fly, of the family Tabanidae, noted for buzzing about animals and sucking their blood.

3. 3

   One who upsets the status quo by posing upsetting or novel questions, or attempts to stimulate innovation by proving an irritant.

4. 4

   One who merely irritates without making useful suggestions.

5. 5

   A bloodsucker; a person who takes without giving back.

   “He's a regular gadfly and takes advantage of his friend's generosity.”