Latin, "horrible to say, dreadful to relate," from neuter of horribilis (see horrible) + ablative supine of dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly").
Entries linking to horribile dictu
horrible adj.
c. 1300, "dreadful, terrible," from Old French horrible, orrible (12c.) "horrible, repugnant, terrifying," from Latin horribilis "terrible, fearful, dreadful" (source also of Spanish horrible, Portuguese horrivel, Italian orribile), from horrere "be terrified, bristle with fear, shudder" (see horror). Used as a mere intensifier from mid-15c.
*deik-
Proto-Indo-European root meaning "to show," also "pronounce solemnly," "also in derivatives referring to the directing of words or objects" [Watkins].
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dic- "point out, show;" Greek deiknynai "to show, to prove," dike "custom, usage;" Latin dicere "speak, tell, say," digitus "finger," Old High German zeigon, German zeigen "to show," Old English teon "to accuse," tæcan "to teach."