Po-faced means having an assumed solemn, serious, or earnest expression or manner; piously or hypocritically solemn. It can also mean expressionless or emotionless, perhaps derived from poker face meaning the ability to avoid showing any reaction to the cards you have been dealt.
When he tries to tell you where you’ve gone wrong, he can be such a po-faced twat.
I was playing cardswith the ladslast night, couldn’t read Charlie at all, totally po-faced.