Bread and butter
WordNet
noun
(1) The financial means whereby one lives
"Each child was expected to pay for their keep"
"He applied to the state for support"
"He could no longer earn his own livelihood"
WiktionaryText
Noun
bread and butter
- Bread spread with butter.
- That which is central or fundamental, as to one's business, survival, or income; a staple or cornerstone.
- They will do some machining if you ask them, but sheet metal has always been their bread and butter.