Quite simple; mind + boggle = mind-boggling.
Originating from Douglas Adams book: The Hitchhiker's Guide to the Galaxy.
"The Hitchhiker's Guide to the Galaxy says that if you hold a lungful of air you can survive in the total vacuum of space for about thirty seconds. However, it does go on to say that what with space being the mind-boggling size it is the chances of getting picked up by another ship within those thirty seconds are two to the power of two hundred and seventy-six thousand, seven hundred and nine to one against."
1. The popularity of the "Tea Party Movement" is mind boggling.
2. "Progressivism is the cancer in America and it is eating our Constitution"
"I'm thinking about killing Michael Moore, and I'm wondering if I could kill him myself, or if I would need to hire somebody to do it. ... No, I think I could. I think he could be looking me in the eye, you know, and I could just be choking the life out. Is this wrong?"
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).