From Japanese doya-gao (ドヤ顔, どやがお). A smug face made after a win or something epic (or not). Doya-gao,
because of this nature, tend to be
irritating.
This word started in Kansai stand-up comedians, and became popular in the M-1 Grand Prix 2006, a manzai tournament. It was adopted quickly on
the Internet, to the point where they had a whole TV program named with the word.
The Germans have a distinct word for it. It is "Backpfeifengesicht", which is a compound word that means "a face badly in need of a fist". Or a crowbar.
See also trollface, bitch slap, Umad? and Backpfeifengesicht.