The word "master" means someone/thing that has the capability of controlling things, like "Mastered", "Master Degree" etc.
But in most Git contexts, "master" is just "one of the breach that we hand picked to put our finest results in", that's not mastering anything, it just means "if you know what's best for you (or not), just use this one".
Another similar wording is in IDE hard drives. Remember the fun time where you can to setup jumpers before your secondary IDE hard drive would work? Yeah that secondary drive is called "slave". I'm still confused why the first drive must be called "master" since I never see it whipping any other drives to make them work harder or really doing anything that's remotely controlling.
The computer guys in the old times really have a weird taste in naming things.