C17: from Latin horrendus fearful, from horrēre to bristle, shudder, tremble; see horror
Word Origin and History for horrendous
adj.
1650s, from Latin horrendus "dreadful, fearful, terrible," literally "to be shuddered at," gerundive of horrere "to bristle with fear, shudder" (see horror). Earlier form in English was horrend (mid-15c.).