god-awful (adj.) Look up god-awful at Dictionary.com
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.