Ripped from the interwebs... "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 [Epiphany, Jan 6th in the UK - ed.], which is the end of the Christmas holidays, when farm laborers returned to the plough. via - Encyclopedia of Word and Phrase Origins.
Had never heard of this Farmers Arms stuff till recently. Good potted history by FionaLake describing the succession of English china makers who since 1800s have produced various cups, mugs, jugs and pitchers with this agricultural toast/motto inscribed. Pristine examples can be expensive but they are all over eBay / Worthpoint etc. The design sort of reminds me of Jack Straws. Anyone??? Perfect gift for the rural man-who-has-everything.