Седьмая вода на киселе (Sed'maya voda na kisele)

Literally: The seventh water on the kissel

Example: “Он мне седьмая вода на киселе, но мы дружим.” — He's a very distant relative of mine, but we're friends.

Origin: Кисель was originally a thick jelly made from oats. As Russian Wiktionary explains, the oat flour was rinsed in water several times and the starchy sediment of each rinsing made the kissel; after several rinsings no more sediment came, so the "seventh water" had nothing of kissel left in it. A remote relative is just as far removed from the family. Michelson's dictionary quotes it from Saltykov-Shchedrin, Dostoevsky and Melnikov-Pechersky, with variants such as десятая вода на киселе.