From olde English dogs, to immoral women, to weak men, to irritating women, to its prideful reclaiming, to ownership over a woman (there's a theme here), the word "b*tch" has a long and fascinating history, and it's all stored in the archives of the Merriam-Webster lexicography department.
Language is an evolving animal. That's why the world needs lexicographers, to update dictionaries so they reflect the language of the time. This paper trail leaves a fascinating historical record, one that Merriam-Webster lexicographer Kory Stamper decided travel down when tasked with updating the definition of the word "b*tch". Stamper noticed there was no label in the dictionary that marked the word as a pejorative. It has meant a lot of different things since it first came into use, sometime before the 12th century, as a term for female dog. Stamper runs through the history of the people this term has applied to, its varied uses, and the muted, bureaucratic struggle that kept it from being marked as an offensive term until the 1990s.