The words terrible and terrific are related! Terrible was borrowed in the fifteenth century from Old French, and the Old French word was borrowed in the twelfth century from Latin terribilis, meaning "frightful". That comes from the verb terrere, which (much like its other descendant terrify) meant "to fill with fear" and derives from the Proto-Indo-European reconstruction ter, "weak". Terrific also traces to terrere, by means of Latin terrificus, which was pretty much the same as terribilis. Until the late seventeenth century, terrific meant "frightening" as well, but people started using it as an adjective for "great" (sort of like how we can now say that shirt looks terribly good on you). Eventually, the old definition was lost and it took on grander, even more positive connotations, giving us the curious contrast we see today.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.