For totes ser is an abbreviation for totally serious. The for is unnecessary, it simply rolls off the tongue.
Let's say you approach me with a question such as: "Jon, did you shoot that hoe?" Then I would respond "Why yes Jake... I'm for totes ser." As in I am being (for) totally serious.
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