Where did the idiom bread and butter originate from?

That would depend on which way it is being used. There are two ways to use Bread and Butter. The most common way is to use it as an idiom for one's livelihood. That company is your family's Bread and Butter. This simply alludes to the fact that without a means to make a living you can't eat. The second way is the superstitious idea that if something comes between a couple while they are walking one of them says, Bread and Butter. Alluding to the impossible task of separating butter once it has been applied to bread. By saying Bread and Butter it means nothing can come between them. These are common themes and likely started simultaneously in different places.