Meaning of “DoyaGao(ドヤ顔)”

“DoyaGao(ドヤ顔)” means “a smug face” or “a boastful face”.
“Gao(顔)” means face.
“Doya(ドヤ)” is a type of onomatopoeia, and I think, it is originate from the Kansai dialect version of the phrase ‘Douda(どうだ)’ (how’s that?), which is ‘Douya(どうや)’. It is used when showing something with a sense of pride.
This word is slang, but it has already been in use for a long time and has become established.

Example Conversation

A: Are you confident?
B: Huh, why?
A: You’re “DoyaGao(ドヤ顔)” now.

A: 自信あるの？
B: え、なんで？
A: めっちゃドヤ顔してるよ。