aghast: [13] Aghast was originally the past participle of a verb, agasten ‘frighten’, which in turn was based on the Old English verb gǣstan ‘torment’. The spelling with gh did not finally become established until the 18th century, and in fact aghast was the last in a series of etymologically related words in the general semantic area of ‘fear’ and ‘horror’ to undergo this transformation. It seems to have acquired its gh by association with ghastly, which in turn got it from ghost (probably under the ultimate influence of Flemish gheest).
