The funniest word on earth, period.
Saying it (and meaning it) will immediately result in the eruption of laughter from all people in the vicinity.
However, overusing the phrase will result it it losing its magical laughter-inducing powers. It does not always work, unless it is said and absolutely meant.
It originated from a name for the BMW "W" logo.
",,,umm...wubbleyou?" = confused stares
"Wubbleyou." = "HAHAHAHAHAHAHAHAHA from everybody.