Hideous
Middle English: from Old French hidos, hideus, from hide, hisde ‘fear’, of unknown origin.
wiktionary
From Middle English hidous, from Anglo-Norman hidous, from Old French hideus, hydus(“that which inspires terror”), from earlier hisdos, from Old French hisda(“horror, fear”), of uncertain and disputed origin. Probably from Proto-West Germanic *agisiþu(“horror, terror”), from Proto-West Germanic *agisōn(“to frighten, terrorise”), from Proto-Germanic *agaz(“terror, fear”), from Proto-Indo-European *h₂egʰ-(“to frighten”). Cognate with Old High German egisa, egidī(“horror”), Old English egesa(“fear, dread”), Gothic 𐌰𐌲𐌹𐍃( agis, “fear, terror”).
Alternative etymology cites possible derivation from Latin hispidosus(“rugged”), from hispidus(“rough, bristly”), yet the semantic evolution is less plausible.
etymonline
hideous (adj.)
c. 1300, "terrifying, horrible, dreadful," from Anglo-French hidous, Old French hideus, earlier hisdos "hideous, horrible, awful, frightening" (11c.; Modern French hideux), from hisda "horror, fear," perhaps of Germanic origin. The old guess that it comes from Vulgar Latin *hispidosus, from Latin hispidus "shaggy, bristly," "presents numerous difficulties" [OED] and seems now to be generally discredited. Meaning "repulsive" is late 14c.
