It comes from the Spaghetti Western 'Preparati la bara!', one of the classic Django movies. Musicians & arrangers like me will be interested to hear the original and divine the progression of DJ Dangermouse's sampling & remixing of elements to bring us the final song, along with Cee-lo's masterful lyrics of un-meaning. Also worth listening to are the several covers, listed on the linked page. And for those who haven't yet seen it, here is the official video for "Crazy", from YouTube (fun game is counting the images of mice that appear).
The Evolution of Gnarles Barkley's "Crazy" - you've all heard it. A song that (nearly) everyone seems to dig. For once, a decent song gains the attention of the hype machine. But have you heard "Nel Cimitero Di Tucson", which is the original form of "Crazy"?