Filled with shock, horror, or amazement; struck with terror or deep concern.
From Middle English 'agast,' past participle of 'agas' (to terrify). The 'a-' is a prefix from Old English 'on,' and 'gast' comes from Old English 'gæstan' (to terrify), related to 'ghost' in the sense of spiritual terror.
The words 'aghast,' 'ghost,' and 'ghastly' are actually cousins—they all come from the same root about being terrified, with 'ghost' originally meaning something frightening rather than just a spirit, showing how words can drift in meaning while keeping family resemblances.
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