|My most favorite written farewell and the inspiration on my knot tying
My understanding is that phrase originates from the old square rigger sailing days. Since ships could sail only slightly up wind, and beating to the wind was arduous work, most sailing was done on a reach or down wind. Also, too little wind and too much wind were problems. So a fair wind was enough wind to fill the sails smartly to push the ship at good clip. Following seas indicated water current abaft the stern, helping to push the ship in the direction you wanted to go. Combine the two and you have the best possible sailing conditions.
Fair winds and following seas,