cash cow

noun Informal.

1. any business venture, operation, or product that is a dependable source of income or profit.

noun

1. a product, acquisition, etc, that produces a steady flow of cash, esp one with a well-known brand name commanding a high market share

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.