Seras is a well known micro-celebrity on anitwitter, he has a tendency to sent weird things to people and so the phrase for when you get something weird from him "My day be so fine, then boom, Seras" was born
Seras: *sends video of guy fucking a toaster*
Everyone else: My day be so fine, then boom, Seras
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