(n.) 1. A person, action, or behavior that is beyond explanation; also "to pull a McPoo" (see example 1)
2. A very quirky, strange, or bizzare person who is difficult to read. (see example 2)
1) "When I told Bill he had just screwed the pooch, I never thought that he would actually do it. What a McPoo!" or "I never expected Bill to do that; he really pulled a McPoo when he screwed the pooch."
2)"Bill majored in drama in college, has feminine tendencies, and acts ambiguously gay. However, he has had a girlfriend for two years to whom he recently became engaged. I don't understand this guy; he is a real McPoo."