from The American Heritage® Dictionary of the English Language, 4th Edition
- interj. Slang Used to indicate the startling abruptness of a sound, action, or event: "The alarm goes off and—whammo!—we're all at our assigned stations” ( Meg Greenfield).
from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. used to emphasis the suddenness of an event
Alteration of wham.(American Heritage® Dictionary of the English Language, Fourth Edition)
From wham (Wiktionary)