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.