Actually, it’s spelled “D’oh”!
I’ve always read the last panel as Xavier saying “D’oh” (like Homer on The Simpsons) plus a “t”… probably should have written it more like that. Oh well. I know it’s not a popular opinion, but I’m not much of a Simpsons fan. I respect the Simpsons for all they’ve achieved, and the impact they’ve had, but I like Futurama a LOT more.
Glen speaking the “Doat language” certainly got their attention though, didn’t it?
