Noun. Colloquial, says Oxford English Dictionary, and chiefly American:
A curse or hex, or any powerful negative influence.
The word is derived from "wham," meaning a heavy blow, so a whammy refers to anything that can deal such a blow (it doesn't have to be supernatural).
OED puts the first usage in 1940, in J.R. Tunis' juvenile baseball novel, The Kid from Tomkinsville: " Interest round the field now centered in the Kid's chances for a no-hit game... On the bench everyone realized it too, but everyone kept discreetly quiet on account of the Whammy. Mustn't put the Whammy on him!"
In addition to citing the word's appearance in the Li'l Abner comic strip (see double whammy), OED notes the word's appearance again in another baseball novel, Bernard Malamud's The Natural in 1952: "They were afflicted with more than the usual number of hexes and whammies and practised all sorts of magic to undo them."
The term is often found in the sports pages of American newspapers in conjunction with "Miami," because of its obvious rhyme: "Team hopes to put whammy on Miami." or "Remember the 'The Whammy in Miami' (the Washington Huskies 38-20 upset victory over the undefeated-at-home Miami Hurricanes in the 1994 Orange Bowl)?" In this latter usage, where "whammy" refers to a particular instance of the hex/curse, it means a "setback" or "disaster." More rare, although not unknown, is its use as a verb, e.g. a sports fan invoking a whammy might cry: "Whammy Miami!"
Sources:
Crowley, Melanie and Mike Crowley. Take Our Word For It Webzine. Issue 59. 1 November 1999. <http://www.takeourword.com/Issue059.html> (9 October 2002).
"Whammy." Oxford English Dictionary, Second Edition. 1989.<http://dictionary.oed.com/cgi/entry/00284061> (9 October 2002) | 
