While the first that comes in mind is the "master copy" terminology of the recording industry, git's "master" terminology in fact comes from BitKeeper. In BitKeeper, there are apparently master and slave branches, where master refers to the source of truth where all the slaves pull their info from.
Hmm there's also something about he who picked the name (which wasn't Linus apparently) meant "master" as in e.g. "master recording", see the Twitter tweets in the comments to that answer. And that he doesn't remember if maybe he was influenced by BitKeeper, was 15 years ago
Since there's no "slave" in git (the master is not special, simply the default branch when creating a repo), and there's the already established terminology of "master copy" or "master recording", it makes sense to not associate git's terminology usage with slavery. I originally posted the BitKeeper story because I had no idea either.
Although I don't think anyone ever seriously thought that "master" branch meant the "lord" branch.