from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Wearing a particularly stern and disapproving expression; humorless; priggish.
- adj. Poker-faced.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. humorless and disapproving
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”. (Wiktionary)