Before there were taps, when the pump would not draw, you poured a dipper of water down it first. That dipper has a name: 마중물, majungmul.
마중 on its own means going out to meet somebody who is on their way to you. So: the water that goes out to meet the water.
It looks like the wrong move — giving water to the thing you are trying to take water from. But the leather in a dry pump will not seal, and until it seals it can lift nothing. Wet it and the whole underground weight of the well comes up and keeps coming.
One dipper out. The well back.