late 14c., "causing terror, awe, or dread; frightful," from Old French terrible (12c.), from Latin terribilis "frightful," from terrere "fill with fear," from PIE root *tros- "to make afraid" (source also of Sanskrit trasanti "to tremble, be afraid," Avestan tarshta "scared, afraid," Greek treëin "to tremble, be afraid," Lithuanian trišėti "to tremble, shiver," Old Church Slavonic treso "I shake," Middle Irish tarrach "timid"). Weakened sense of "very bad, awful" is first attested 1590s.
