a masterbathon is a day that you stay home from school "sick" so you can beat off so many times that you are dry and chaffed and can't walk so the next day you have to stay home and do it all over again
Not an example of usage, examples of actual masterblathering:
"You don't have to tell me how cool I am, listen to ME tell you how cool I am!"
"Did you see me at Madison's party? Everybody was waiting for me to do something cool, I know, but I made them wait until Madison was right next to me. Did you see it? I was SO AWESOME, dude!"
"So I watched this movie once called "City on Fire" at the video store they let me work at and I thought "Shit, nobody's seen this! If I can get Harvey Keitel and that funny-looking guy from Fargo I can totally rip off my first movie!"