"Master" is used in lots of circumstances as the "authoritive version". You've heard of films and music being "remastered"? Well that's because the version all the production copies are copies of is the "master". "Remastering" is the process of remaking the master.

This is the usage in git. It has nothing to do with slaves.

A master is the original of something. For example, in the music industry when they printed records, they would print a master record from which duplicates would be made for manufacture. They still use the phrase mastering to describe finalising music in production.

Master copy has become widely used to mean the original version of something from which other copies are made.

Apparently the whole concept dates back to the renaissance where artists would learn by copying the works and techniques of a Master artist. Master in that context is referring to someone who is the best at what they do (the most famous renaissance artists are still referred to as the masters), which presumably derives from the honorific Master used to refer to a male teacher in English.

While perhaps understandable, moving away from Master to Main is based on ignorance of the fact the word has a totally different meaning and origin to master/slave. Ironically master/slave is also used in engineering and computer hardware to describe the relationship between a controlling piece of hardware and it's subordinates, but that is nothing to do with the source of truth use of master which comes from the concept of a master being the best at something (or now the best version of something).

Git borrowed the term “master” from BitKeeper, the proprietary tool the Linux kernel used before Git existed.

In BitKeeper, a “master repository” referred to the authoritative repository in a set of distributed clones. When Linus Torvalds created Git in 2005, he followed that naming convention and called the primary branch “master”.