Backstaff :: Backstaff (n.) An instrument formerly used for taking the altitude of the heavenly bodies, but now superseded by the quadrant and sextant; -- so called because the observer turned his back to the body observed..
Backstair :: Backstair (a.) Private; indirect; secret; intriguing; -- as if finding access by the back stairs.
Backstay :: Backstay (n.) A rope or stay extending from the masthead to the side of a ship, slanting a little aft, to assist the shrouds in supporting the mast..
Backstay :: Backstay (n.) A rope or strap used to prevent excessive forward motion.