bread and butter is a noun meaning "the financial means whereby one lives". It carries one recorded sense.
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”
Definitions, synonyms and semantic relations for bread and butter come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.