windmill

1a :  a mill or machine operated by the wind usually acting on oblique vanes or sails that radiate from a horizontal shaft; especially :  a wind-driven water pump or electric generatorb :  the wind-driven wheel of a windmill

2 :  something that resembles or suggests a windmill; especially :  a calisthenic exercise that involves alternately lowering each outstretched hand to touch the toes of the opposite foot

3 [from the episode in Don Quixote by Cervantes in which the hero attacks windmills under the illusion that they are giants] :  an imaginary wrong, evil, or opponent —usually used in the phrase to tilt at windmills