The origin of the word Sod Off

Sod-off is thought to derive "from the Middle Ages with the turf-dwellers of Romford (Essex on the poor side of London). These hard working peasant folk lived in low grass covered houses on the banks of the Thames.
In order to enter or leave one of these Squatts as they were known, a piece of turf or sod would be lifted away from the door opening. The term "sod off" was thus coined to mean "The door is open, feel free to leave".
This term eventually replaced such phrases as "Well, don't let me keep you", "My, is that the time?" and "We must have lunch together one day".