What is the X in X-mas?
c.1400, from power (n.) + -ful. Meaning "of great quality or number" is from 1811; colloquial sense of "exceedingly" (adv.) is from 1822. Related: Powerfully. Thornton ("American Glossary") notes powerful as "Much used by common people in the sense of very," along with monstrous 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).