Of Old English origin, shortening of "Good Speed," and contrary to popular belief has nothing to do with God. People would say this to others who were leaving on a long and perilous journey, wishing them success, but more importantly wishing for their return.
It's a dangerous and unforgiving sea, yet the sailors had no fear. They set course due West with no way to measure longitude and only crude instruments to determine latitude by the stars. Their friends and family at the docks had only one thing to say to them - "Godspeed."