Neither. My theory is that his last name is Spike. You see, in my point of view, Wario's father was Foreman Spike, who married Mrs. O`Dove, then they had Wario, divorced, and O`Dove had Waluigi, who became a chef before being a plumber and competing in Mario Tennis. So in that case, Waluigi's...
My two favorite plumbers in the Mario series are Luigi and Waluigi, though I chose to only display that I like Luigi because LuigiMan sounds better than WaluigiMan IMO. The "Man" is because I have a capitalized Man at the end of my name on all the forums I've joined, simply so I can remember all...