c. 1400, pouerful, "mighty, having great strength or power," from power (n.) + -ful. Meaning "of great quality or number" is from 1811; colloquial sense of "exceedingly, extremely" (adv.) is from 1822. Related: Powerfully. Thornton ("American Glossary") notes powerful, along with monstrous, as "Much used by common people in the sense of very," and cites curious expressions such as devilish good, monstrous pretty (1799), dreadful polite, cruel pretty, abominable fine (1803), "or when a young lady admires a lap dog for being so vastly small and declares him prodigious handsome" (1799).
This gross perversion is common in several of the Western counties of Pennsylvania, to which region I had supposed it was limited. A gentleman informs me, however that it is not unfrequent in the South, and that he has even heard it yoked with weak, as, A powerful weak man. [Seth T. Hurd, "A Grammatical Corrector; or, Vocabulary of the Common Errors of Speech," 1847]