A dollar for a doughnut

What does the phrase "bet a dollar for a doughnut" mean and what is the history of this phrase?

It's actually "bet dollars to doughnuts" and it's been discussed here a few times before. It means you are so sure of something that you would bet valuable dollars to doughnuts which cost very little (or at least they cost very little, perhaps a dime a dozen, when the phrase was coined).