"the Big Dipper;" M.E. septentrioun (1530s, but septentronial "northern" is attested from late 14c.), from L. septentriones (pl.) "seven plow oxen," from septem "seven" + trio (gen. triones) "plow ox," from stem of terere (pt. tritus) "to rub" (see
throw). Cf. also
Charles's Wain.