1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): back and fill
Fig. to act indecisively; to change one’s direction repeatedly; to reverse one’s course. (Originally nautical, referring to trimming the sails so as to alternately fill them with wind and release the wind, in order to maneuver in a narrow space.) • The president spent most of his speech backing and filling on the question of taxation. • The other candidate was backing and filling on every issue, depending on whom she was addressing.
McGrawhill's American Idioms And Phrasal Verbs