Φέρνω νερό στο μύλο κάποιου

Literally: Bring water to someone's mill

Example: “Με τις επιθέσεις του στον υπουργό, φέρνει νερό στο μύλο της αντιπολίτευσης.” — With his attacks on the minister, he's playing into the opposition's hands.

English equivalent: To play into someone's hands

Origin: Grounded in the everyday mechanics of a genuine watermill: without a steady flow of water to turn its wheel, a mill simply couldn't grind grain at all, a dependency preserved directly in the related Greek proverb δίχως νερό ο μύλος δεν αλέθει — without water, the mill doesn't grind. "Bringing water to someone's mill" treats any contribution, action, or resource that advances a person's goals the same way a mill owner would treat water delivered to their wheel: a direct, practical assist toward getting the intended result. The idiom's now-common ironic edge — helping an opponent's cause while believing you're actually working against it — extends that same image into a sharper, more self-defeating context.