daf bit: Nedarim 4
Against that backdrop we will attempt to study nedarim. This looks like it could be a complicated tractate; I'll do my best.
The first mishna begins by listing some different types of nedarim, one of which is the Nazarite vow. A Nazarite had to refrain from grape products, cutting his hair, and having contact with the dead for the period of the vow, which is usually not permanent. The g'mara tells us: the Master said that just as in other vows, for a Nazarite vow the father can annul the vow of his daughter and the husband can annul the vow of his wife. An objection is raised: this is obvious -- why are you telling us this? Why should these be special that we would think otherwise? The g'mara answers its objection: perhaps with other vows we might say that he can annul because the vow is permanent while, for a Nazarite vow, it goes away on its own soon enough so there is no need for him to have his privilege. To cut off that line of reasoning, the g'mara tells us outright that a woman's Nazarite vow can be annulled. (2a mishna, 4b g'mara)
The part of this that I found most surprising is that women might take Nazarite vows. I guess because at the end the Nazarite shaves his head, I didn't expect that.
I also am still pretty unclear on why people would do this.