The expression “to back and fill” means to shilly-shally; to be vacillating or irresolute; to assert and deny, hem and haw; not to know if one is on one’s head or heels.
Originally this was said of ships, of sailing ships especially, attempting to negotiate a narrow channel when wind and tide were adverse and there was no room for tacking.
Under such conditions a vessel may be worked to windward by keeping it broadside on to the current in mid-channel by counter-bracing the yards or keeping the sails shivering, that is, alternately backing and filling the sails.
The progress of the ship is thus alternately backward and forward, in herringbone pattern; hence, anything that appears to do nothing more than to recede and advance, to vacillate, is said to back and fill.
