An expression of disinterest. Usually followed by a number -- the higher the number, the greater the lack of interest -- and, occasionally, a reference to Sulu, the helmsman of the Enterprise.
As this phrase is a reference to the original Star Trek series, it shall be generally understood that 9 represents the highest normal whoop factor. Anything higher indicates an excessive whoop factor -- and, by extension, an extreme lack of interest.
Figure skating on TV? Whoop Factor 9, Sulu!