Vacillate, be undecided, as in This measure will never be passed if the town meeting continues to back and fill. This term comes from sailing ships, where it signifies alternately backing and filling the sails, a method used when the wind is running against a ship in a narrow channel. The sail is hauled back against the wind and braced so that the tide or current carries the ship forward against the wind. Then the sail must be swung around and filled, to keep the ship on course. The term's figurative use for indecisiveness dates from the mid-1800s.