Shawn worked at a casino, right? Of course, he would've heard cursing in more languages than one could imagine! I, for one, can imagine him uttering a soft yet disrespectful "baaaahh-ka" ("idiot" - but much worse - in Japanese) or "ksoo!" ('crap!' - If you're an anime fan, you'll recognize these words right away...).
S: Baaaah-ka. [Idiot.]
G: Wait a minute, Shawn. I know that word from somewhere.
S: Where did you hear this from? I learned it from some Japanese customers at the casino when they lost all their money.
G: Oh, I remember. Anime characters say it all the time...
S: Gus, Gus... You should really wean yourself away from those - the comic books were bad enough for your image.