Definitions for po-facedˈpoʊˌfeɪst
Random House Webster's College Dictionary
Chiefly Brit.having an overly serious demeanor or attitude; humorless.
Category: Common Vocabulary, British
Origin of po-faced:
humorless and disapproving
Wearing a particularly stern and disapproving expression; humorless; priggish.
Origin: Probably from po, abbreviated slang based on French pot de chambre (chamber pot), after the distasteful expression anyone would adopt upon being presented with a full one. Perhaps similarly from pooh; perhaps influenced by poker-faced. Also possibly “poor-faced”.