Double whammy
What's the meaning of the phrase 'Double whammy'?
A double blow or setback.
What's the origin of the phrase 'Double whammy'?
A whammy was originally an evil influence or hex. It originated in the USA in the 1940s and is associated with a variety of sports. The first reference to it in print that I can find is in the Syracuse Herald Journal, October 1939:
"Nobody would have suspected that the baseball gods had put the whammy on Myers and Ernie when the ninth opened."

"Shore there's only one way to beat Joe Louis ... No man can lick 'im, it takes a syndicate and that's what I got. I've been taking a course in hypnotism from the famous Professor Hoffmeister of Pennsylvania. When I gave my big police dog the evil eye like this he liked to collapse, went out and nearly got himself killed by the neighbour's pet poodle pooch. Professor Hoffmeister says I don't get the double whammy to put on human beings until Lesson 9."
Ross was well-known for his tall tales and flowery language. It is quite possible that he coined the term in that interview.
'Double whammy' is often associated with Al Capp's Li'l Abner cartoon strip, which featured the phrase several times. In that it referred to as an intense stare which had a withering effect on its victims; for example, this piece from Li'l Abner July 1951:
"Evil-Eye Fleegle is th' name, an' th' 'whammy' is my game. Mudder Nature endowed me wit' eyes which can putrefy citizens t' th' spot!. There is th' 'single whammy'! That, friend, is th' full, pure power o' one o' my evil eyes! It's dynamite, friend, an' I do not t'row it around lightly! ... And, lastly - th' 'double whammy' - namely, th' full power o' both eyes - which I hopes I never hafta use."

See other phrases that were coined in the USA.
