Shat is the past tense of Shit which is the present tense of crap, so, i shat my pants is saying i shit my pants, which means i crapped my pants, which means i took a poop in my pants, which means I dumped a lump
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi