Bread and Butter
There are two meanings associated with this idiom. The first refers to your job or vocation and it comes from the thought that you can’t eat if you aren’t able to make a living. No job means no “bread and butter” on the table.
The second meaning refers to a couple that are the bread and butter together, meaning they can’t be separated. This comes from the notion that once butter is melted onto bread, it is nearly impossible to separate the two elements.
The expression was first used as an exclamation when two people walking side by side were momentarily separated by something coming between them. The earliest citation of this comes from The Federal Writer Project “Guide to Kansas” which was published in 1939, where “bread and butter” was described as an incantation among schoolchildren in the area.