Haha, clueless. I'll explain once and once only. You're on your own after that.
"woman" originally meant a female dog but has been used to refer to women in a derogatory fashion since the 15th century, according to the Oxford English Dictionary.
It was originally used against women deemed to be too sexually active because female dogs give birth to many puppies. During the age of women’s suffrage in the early 20th century, "woman” became the word for women who defied the patriarchy and became doubly loaded with negative connotations. Damn those uppity women who love sex and want basic equality!
“woman” isn't just an insult because it means "promiscuous" and "uppity" - it's also an insult because for the user it means “woman.”
As for women using it as well (I cannot believe I'm having to explain this to you), the word “woman” is for women and women’s mouths only, as they're targeted group of the slur. If she chooses to reclaim it, that is each woman’s decision for herself. It's not for men.