A fork of Rural Dictionary
It is a measurement of time longer than one hour, approximately an 1hr10mins to 1hr30mins long. It is a guesstimation.
Its origin is that metric measurements are slightly off from standard measurements: a meter is slightly longer than a yard, a liter is smaller than a quarter...etc
You are driving and you are 80 miles from your destination, it will take you a metric hour to get there.