god-awful (adj.)youdaoicibaDictYouDict[god-awful 词源字典]
also godawful, according to OED from 1878 as "impressive," 1897 as "impressively terrible," but it seems not to have been much in print before c. 1924, from God + awful. The God might be an intensifier or the whole might be from the frequent God's awful (vengeance, judgment, etc.), a common phrase in religious literature.[god-awful etymology, god-awful origin, 英语词源]
impressive (adj.)youdaoicibaDictYouDict
1570s, "capable of being easily impressed," from impress + -ive. Meaning "capable of making an impression on the mind or senses" is from 1775. Related: Impressively; impressiveness.