Green’s Dictionary of Slang

arrogant, stand-offish, humourless;' also as adv. and personnified as po-face.

1934		C. Lambert Music Ho! (1948) 138: I do not wish, when faced with exoticism, to adopt an attitude which can best be described by the admirable expression ‘po-faced’ .
1943		J. Lees-Milne Ancestral Voices diary 17 Oct. (1975) 257: The jokes were bawdy beyond words. Sandwiched as I was between the two po-faced ladies-in-waiting I did not dare laugh.
1959		A. Sinclair My Friend Judas (1963) 130: Wise up wise guy. Wisdom’s more than a po-face and a big vocabulary.
1960		L. Davidson Night of Wenceslas 163: [T]he characteristic poe-faced look of the S.N.B. [i.e. secret police].
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 16: Now I’m stuck with this po-faced sheila.
1967		J. Orton Diaries (1986) 11 Jan. 57: I spotted Michael Codron. Dark-suited and a little po-faced.
1975		A. Bleasdale Scully 160: She was still po-faced.
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 205: Billy suppressed a groan and nodding his head very po-faced, quickly turned away.
1985		J. Morrison Share House Blues 101: Will you please inform your inspector, Mr Poface, that the next time he sets foot in my house I will scald him?
1992		R.G. Barrett Davo’s Little Something 41: She nodded to where her po-faced girlfriend was standing.
1993		I. Welsh Trainspotting 168: He’s escorted away by a po-faced labdick.
2004		Guardian 2 Oct. 24: Maybe I’m a po-faced party pooper.
2011		Week 8 Jan. 25/1: Why be so po-faced, said Sam Marlowe.
2016		Guardian Guide 21 May 10/3: Pop is having a bit of a po-faced moment. Most artists take themselves very seriously.