Uber simply translates to "over", "above" or "across" in German.
Famous philosopher Friedrich Nietzsche use Übermensch to mean the ideal superior man.
Today, the OED lists uber as “denoting an outstanding or supreme example of a particular kind of person or thing," and in the Urban dictionary, uber is said to mean "the ultimate, above all, the best, top, something that nothing is better than."
Uber did start as UberCab. They changed there name after having regulatory issues related to being an unlicensed taxi service. UberCab is a hybrid name (intriguing and descriptive)--Uber is simply an intriguing name. (More about naming Styles.)