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."
fantastic, tremendous, crazy
that car is mind-boggling