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1.
The term one uses to refer to almost-finalized source code. It compiles and runs without any complaints, and only needs to be tested for minor logic errors. The transition from master beta to golden code should only take a few minutes of half-hearted debugging, but invariably requires an all-nighter to work out the most prominent segfaults.
Student 1: Hey man, have you finished your part of the project yet?
Student 2: I've got the master beta saved in my home directory.
Student 1: Cool beans. Let's debug it after class.
Student 2: You're bringing the soda this time.
by aero March 14, 2003
 
2.
One who engages in autoerotic stimulation, i.e., one who masturbates.
Look at that master beta getting his rocks off!
by master_masterbeta November 28, 2004
 
3.
A person who spends an un-healthy amount of time playing a beta (trial) version of a game, usually with the hopes it will make them cool/interesting.
Guy 1: Hey how's the Gears of War Beta?

Guy 2: Awesome! I'm already ranked 54 in the world for deathmatches!

Guy1: Fucking Masterbeta.

Person: You hear the Phantasy Star beta got shut down?

Other Person: I know! It's terrible! I would play that shit all the time with my friends! We kicked the D-Ragon's ass all the time!

Person: You guys are fucking Masterbeta's.
by Steve&Martia April 19, 2011
 
4.
The winning beta fish in a fight.
Hey man wanna get together and find out who the master beta is?
by Cass Ass May 10, 2006
 
5.
descriptive noun. the term one is called when they master bet.
by me March 14, 2003