I am guessing that it is a short form of "God speed the plow." Does anyone know for sure?
GOD SPEED THE PLOW -- "God speed the plough, 'a wish for success or prosperity,' was originally a phrase in a 15th-century song sung by ploughmen on Plough Monday, the first Monday after Twelfth Day, which is the end of the Christmas holidays, when farm laborers returned to the plough. On this day ploughmen customarily went from door to door dressed in white and drawing a plough, soliciting 'plough money' to spend in celebration. "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997).