A source of money, esp a generous one: But all this leaves The New Republic Inc without a cash cow/ For a fairly blatant cash cow, Pisces Iscariot delivers some fine milk(1970s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with cash cow
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.