The surf-licious way to say "yeah." May or may not (judging by the context) include a connotation of "your an idiot!" or "HELL yes!" Note: that when emphasising your attack on your buddy's foolish remark, you should enunciate the "ch" part of the word, possibly even holding it for a moment longer
"So are we gonna go snowboarding tomorrow?" "Chya! (We are!)"
"So wait is the sky REALLY blue?"
"Chya! (Ya retard!)"