ghastly:  Despite its similarity in form and sense, ghastly is not related to ghost. It was formed from the Middle English verb gasten ‘terrify’, which may have been a descendant of the Old English verb gǣstan ‘torment’ (source of aghast). The spelling with gh-, based on ghost, was first used by the 16th-century poet Edmund Spenser, and in due course caught on generally. => aghast
c. 1300, gastlich, "inspiring fear or terror, hideous, shocking," with -lich (see -ly (2)) + gast (adj.) "afraid, frightened," past participle of gasten "to frighten," from Old English gæstan "to torment, frighten" (see ghost (n.)). Spelling with gh- developed 16c. from confusion with ghost. Middle English also had gastful in the same sense, but this is now obsolete. Sidney and Shakespeare also used ghastly as an adverb. Related: Ghastliness.