Definition of: whammy

(hwam′ē) noun plural ·mies U. S. Slang
1. A gesture made by extending in parallel the index and little finger from the closed fist and pointing toward the person or object intended: an ancient form of hexing (mano cornuta, sign of the horns). If the fingers of both hands are used, the fists being in contact, it is a double whammy.
2. A jinx; hex: to put the whammy on someone. [<wham, colloq. interjection imit. of the sound of a hard blow]