milch cow
Synonym discussion
Sense: noun/1
All four describe a source of money, but the feeling differs. A milch cow is usually disapproving: it suggests someone or something being squeezed for cash again and again, often unfairly, and the term is chiefly British. A cash cow is more neutral and far more common, especially in American English; it normally praises a product or business that brings in steady profit, like a hit film for a studio. A gold mine stresses how rich and plentiful the source is, not the repeated 'milking'. A money-spinner is British and informal, simply meaning something that earns a lot of money. Compare: 'The charity became a milch cow for its directors' (critical) versus 'The phone line is the firm's cash cow' (approving).
Etymology
Milch is an old English word meaning 'giving milk', from the same Germanic root as the word milk. A milch cow first meant simply a cow that was being milked. Because such a cow could be milked over and over, the phrase later grew a figurative sense: any person or thing that others can keep 'milking' for money.