What do a serious look and a dishpan have in common? Plenty. "Pan" has been used since the 19th century to mean "the face," possibly because the human face is broad and shallow like a dishpan. By adding "dead," meaning lifeless, a deadpan look would therefore be so broadly unanimated as to have not even a twinkle of expression. In the last 200 years, this common word for poker-faced has moved beyond the visage: A deadpan news story is similarly lifeless, and one should not expect to detect any humor or feeling in someone's deadpan remarks.
SOURCES: The Encyclopedia of Word and Phrase Origins, by Robert Hendrickson; Webster's Dictionary; The World Book Dictionary; The Random House Dictionary of the English Language.
(c) Copyright 2001. The Christian Science Publishing Society