"causing or attended by great fear, dreadful, awful," 1560s, from Latin dirus "fearful, awful, boding ill," a religious term, which is of unknown origin. Apparently a dialect word in Latin; perhaps from Oscan and Umbrian and perhaps cognate with Greek deinos "terrible," Sanskrit dvis- 'hate, enmity, enemy," from PIE root *dwei-, forming words for "fear; hatred."
