any business venture, operation, or product that is a dependable source of income or profit.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a product, acquisition, etc, that produces a steady flow of cash, esp one with a well-known brand name commanding a high market share
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.