caribou nibbling at the croquet hoops

Something I've been wondering about: on one of the episodes of Ramsay's Kitchen Nightmares he says, referring to a situation that's totally fubar-ed, that things have gotten "pear-shaped." What in the what now? Googling, I've found several citations but they all fall frustratingly short of certainty. I mean, what it means was immediately clear from the context. But how did it come to mean what it means?