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.)