n. a dependable source of money; a good investment. : Mr. Wilson turned out to be the cash cow we needed to start our repertoire company.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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A dependable source of profit, as in The small-appliance division is this company's cash cow. Although this precise term dates only from about 1970, milch cow was used in exactly the same way from 1601.