bread and butter
The essential, sustaining element, as in The quality of the schools is the bread and butter of town property values. This idiom alludes to a basic food, bread spread with butter. [c. 1700]
Means of livelihood, as in John's job is the family's bread and butter. [First half of 1700s]
Ordinary, routine, as in Don't worry about it; this is just a bread and butter assignment. [Second half of 1800s]
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