frightful adj.

Middle English freiht, fright, from Old English (Northumbrian) fryhto, metathesis of Old English fyrhtu "fear, dread, trembling, horrible sight," from Proto-Germanic *furkhtaz "afraid" (source also of Old Saxon forhta, Old Frisian fruchte, Old High German forhta, German Furcht, Gothic faurhtei "fear"). Not etymologically related to the word fear, which superseded it 13c. as the principal word except in cases of sudden terror. For spelling evolution, see fight (v.).

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).