The expression "double whammy" first appeared in a cartoon strip, which was quite popular in America in the 1920s. Al Capp, the author of the comic strip "Li'l Abner" used this expression to refer to an intense stare, which had a withering effect on its victims. It's meaning has undergone a considerable amount of change since then. This is how slang dictionaries define "double whammy" — "double the portion of something, especially something troublesome." When you get a "double whammy" of something, it is usually something really bad. Here is an example:
"We thought he'd come alone, but he gave us a double whammy by bringing his wife and children."
N.B: The original quote is:
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.