Only God knows, but maybe основная жила (о руде) или в переносе главное что-то.

A mother load is a huge source of something - especially something illicit like drugs or stolen goods, money. It refers to one place that's absolutely FULL of it. Say some thieves break into a house where they know there's a lot of money hidden. They search all the rooms and then find one with a strange door. They bust through and see that it's brimming with gold. They say "We hit the mother load!"