From Wiktionary, the free dictionary
melken (“to milk”) + Kuh (“cow”)
Melkkuh f (genitive Melkkuh, plural Melkkühe)
- (idiomatic) cash cow
Declension of Melkkuh [feminine]
- “Melkkuh” in Digitales Wörterbuch der deutschen Sprache
- “Melkkuh” in Duden online