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 *tres- "to tremble" (cognates: Sanskrit trasati "trembles," Avestan tarshta "feared, revered," Greek treëin "to tremble," Lithuanian trišeti "to tremble," Old Church Slavonic treso "I shake," Middle Irish tarrach "timid"). Weakened sense of "very bad, awful" is first attested 1590s.