I think Politese posted what the word master would have meant in the language of the day.
My use of the word "Master" comes from Nietzshe. Not the Bible. "Master morality" is a Nietzschian term. Master and Slave are dichotomies and I think they are supposed to evoke our modern idea of that relationship. A Master is dependent on no one, and the Slave is dependent on someone. I'm convinced Nietzsche was well aware of the unlikely situation that a person is ever not dependent on anyone.
In terms of Jesus amd Christianity I see it more as the alpha male than master - slave. Humans naturally fall into a hierarchical power structure. Historically male.
The Abrahamic god is the ultimate alpha male. The believer puts god at the top and owes allegiance to the alpha male above all else.
The Seattle Seahawks quarterback Russel Wilson says in interviews, 'God first, family second, football third'. It gives the believer a sense of stability and empowerment. Not a master slave relationship.
Nietzsche's Master Morality is a morality of people. Not God. Nietzsche was an atheist. Nietzsche was convinced that humans wrote the Bible. Nietzsche also defends Jesus and attacks Paul.
I think Nietzsche's attitude towards Christianity is the most misunderstood aspect of Nietzsche. "God is dead for we have killed him", isn't us gloating of the death of God. Nietzsche was what he called an "affirmative nihilist". Nihilism just means nothing matters. A true nihilist won't get anything done and has no reason to get out of bed. It's a paralyzing and pointless existence. "Affirmative Nihilism" is when you pick a meaning of life. You elevate something to sacred and use that as a guiding principle for your life. It could be anything. As long as it gets you out of bed, it's good. He's not having a go at Christianity. I think Nietzsche thought Christianity was a perfectly fine type of affirmative Nihilism. Yes, he thought it was a delusion. But we all cling to delusions to get by in life. I think it's not "ha ha, God is dead, for we have killed him". It's "oh, no, God is dead for we have killed him". Nietzsche isn't anti Christian. His project isn't to destroy Christianity. What I think he did say that industrialism and the Enlightenment had, in his lifetime, killed Christianity already. Which explains why we have the buffet style McDonalds Christianity of today. Which has very little to do with Christianity of old.
Another mistake is to see Nietzsche's masters as superior. The übermensch. As far as Nietzsche was concerned, being authentic and true to yourself will lead to endless grief and pain in your life. You will be admired by a little group of fans or others who are similar, but hated by most of society, for not complying. The life of a slave (ie, those with slave morality, not actual slaves) is a much more pleasant life. Less conflict.
Nietzsche's project wasn't to vilify Christians. It was just to describe the underpinning sociological mechanic in plain terms. I think he thought the death of Christianity was a catastrophe for human social cohesion, and would lead to grief and destruction, as the social norms keeping humanity from murdering each other willy nilly would come off.
Interestingly enough Kierkegaard (one generation before Nietzsche) made a very similar argument to Nietzsche. Kierkegaard's apology for Christianity comes down to, "meh... why not? What else are you gonna do?" It's not much a ringing endorsement.
In western culture master would refer to the male head of household, and mistress the female. Brits still recognize aristocratic titles, like lord. Lord of the manor.
From the gospels Jesus wa not a social activist. From the Sermon On The Mount and a few other passages one can infer passivism. The Jesus theme was bear your burden whatever it is and your eternal reward is in heaven.
To me Jesus is schizophrenic or bipolar. He goes from the pacific Sermon On The Mount to rage in the Temple. There is no coherent message in the gospels.
Will the real Jesus please stand up. On picks the Jesus that suits you. The old and new testaments are paradoxical if you take any of it literally. None of it was intended as a coherent moral guide.
I think this is by design. The Bible has a little bit of something for everyone. You can take from it what you will... pretty much. Christians did put in a lot of effort into collating the Bible and spent a bizarre amount of time talking over every aspect of it. A major project for the post-Constantinian church was to get all the Christians on the same page.
I think Jesus in the Bible is schizophrenic for a reason. That's the Jesus we needed at the time. And the continued popularity of Jesus seems to prove the point that he's still needed.