war·ble 2 
(wôrbəl)
(wôrbəl)n.
1.
a. An abscessed boillike swelling on the back of cattle, deer, and certain other animals, caused by the larva of a warble fly.
b. The warble fly, especially in its larval stage.
2. A hard lump of tissue on a riding horse's back caused by rubbing of the saddle.
[Probably of Scandinavian origin; akin to obsolete Swedish varbulde.]