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.