milk cow
/ˈmɪlk ˌkaʊ/noun
A cow bred or kept for milk; a cow yielding milk. Also figurative : a ready or reliable source of income or profit.
Origin
Late Middle English. From milk + cow. Compare West Frisian melkko, Middle Dutch melccoe (Dutch melkkoe), German regional (Low German) Melkkoh, German Milchkuh, Old Icelandic mjólkr-kýr, Old Swedish miolka ko (Swedish mjölk-ko).