A Doomba is an amalgamation of the Roomba; a series of autonomous cleaning devices made by the company iRobot and various weapons such as knives, chainsaws and fireams which are strapped or somehow attached to the machine. The word "Doomba" comes from the merging of the words "Doom" and the previously mentioned "Roomba". This has recently become popular on various Internet sharing sites.
"Hey Johnny, I strapped some knives to your mom's Roomba, let's watch it go around the room."
"Yeah, now it's a DOOMBA"