Definition: An urge to do something inadvisable.
Audrey was walking down the highway when she espied a grasshopper-infested cornfield.
It was the meaning of life. She was a chicken, you see, and her mother had always told her never to eat grasshoppers.
Seized by a fit of cacoethes, she ran across the road and caused a Camry to run into a pink stretch Hummer.
Consternation all round.
(NB: No chickens or Camrys were hurt in the making of this post. However, the Hummer was somewhat less pink in the bumper afterwards.)