Excellent. Superlative. Top notch. Used to express delight, satisfaction, victory, or pleasant surprise.
This comes from an episode of "The Simpsons" in which the geeky and unpopular character Milhouse
enjoys a small number of fortunate happenings, such as finding Ten Dollars. In response to the lucky occurrence, he exclaims "Everything's coming up Milhouse!" The total expression is reduced to "Millhouse" for convenience. Millhouse is a misspelling of Milhouse