† ˈback-ˌstaffObs. [f. back n.] A peculiar kind of quadrant formerly used in taking altitudes at sea, so called because the observer turned his back to the sun. 1627Capt. Smith Seaman's Gram. xv. 73 A Crosse staffe, a Backestaffe, an Astrolobe.1696in Phillips; in mod. Dicts.