vache à lait

Someone repeatedly exploited for money

Just like a farmer keeps milking the same cow, this describes a person or business that keeps getting drained for cash. English has "cash cow" for the commercial version.

Mon oncle se plaint d'être la vache à lait de toute la famille.

My uncle complains about being the one the whole family milks for money.

Alphonse Daudet used it in his 1868 novel Le Petit Chose, where a character grumbles that he can't always be the family's vache à lait, so the image of being milked for money was already well established by the 19th century.