One meaning of the word is "female master." The Latin equivalent would be Domina.
Another connotation is "lover." Not quite what one associates with "Domina."
Or was there a connection between the two made when men took "dominant" women as lovers?
Could one possible instance of this phenomenon be the medieval "courtly love?"
https://history.stackexchange.com/questions/1125/what-was-courtly-love-amour-courtois
The Oxford English Dictionary’s earliest citations for mistress are from the fourteenth century when it meant, very broadly, ‘a woman having control or authority’. It had the sense of ‘a woman loved and courted by a man; a female sweetheart’, with no indication of impropriety, very early, perhaps a mere 100 years after its first recorded use. Almost from the start it seems to have had multiple meanings. That’s hardly surprising given how common a word it must have been. The dominatrix sense, however, doesn’t appear until 1921.
Check out the online etymology.
This term comes from French, as many English terms do, and in French, words vary in gender depending on whether they apply to a man or a women.
A man in charge of a number of domestic staff, workers or pupils would be called "maître" and a woman "maîtresse" (same word but applying to a member of the fair sex).
A woman came to be called "maîtresse" as a romantic metaphor for the influence she held on a man's heart (at that time, this term did not imply -or reject- illicit relationships or even a carnal relationship), just that the woman was loved. The term then came to designate a fiancee and in the 17c a women having sexual relations with a man outside of marriage.
In short, no, the term Mistress did not originally imply a dominant lover and still does not by default, unless the context suggests it, of course.
The meaning of "female master" came first. The second meaning of "illicit lover" seems to follow quite naturally to me. I don't have any documentary historical evidence to back this up, just my intuition, but it seems quite natural that people would think of an illicit lover as dominating (domina + ing) her boyfriend through her seductive powers. He is enslaved to her through his lust and her charms. The usage is not literal but whimsical.
I doubt that it has anything to do with dominant women in the sense of women with forceful personalities, and even less likely that it has to do with women of high social or political status. Maybe I'm applying modern connotations to antiquated usage here, but if a man was having an affair with the queen, I don't think he would call her his "mistress": that would be demeaning to her, and I presume he wouldn't dare.
It is common for men to talk about their wives half-joking as being dominating, even if she is not a very domineering person. When he changes his plans to appease her or consults her before making some minor decision, he often comments on how he "must check with the boss" or follow "she who must be obeyed".
This isn't a new idea. I can't find the quote right now, but I recall reading a quote from an ancient Greek king to his wife that was something to the effect of, "All Greece is ruled by a 2 year-old child. For our son rules you, you rule me, and I rule the Greeks."
