bread and butter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Means of support; livelihood.
• noun The essential sustaining element or elements; the mainstay.

from The Century Dictionary.

• Seeking bread and butter, or the means of living; controlled by material wants and desires; mercenary: as, the bread-and-butter brigade (applied to office-seekers in the United States).
• Eating much bread and butter, as young boys or girls; hence, belonging to adolescence; in the stage of growth: as, she's but a bread-and-butter miss.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Used other than as an idiom: see bread,‎ butter.
• noun idiomatic That which is central or fundamental, as to one's business, survival, or income; a staple or cornerstone.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the financial means whereby one lives