False origin is a selected point on a map's grid system from which the coordinates of any point in the map area are measured. The false origin differs from the true origin in order to exclude negative values from the coordinates. 
A false origin is used in order to make finding map coordinates easier on a map. Usually it is brought down to the lower left of the map so all of the x,y coordinates are positive.
http://www.delorme.com/glossary.aspx, DeLorme Mapmakers.