Neutral: In an Author's Note at the front of his novel The Matchmaker of Kenmare, Frank Delaney writes that the word neutral is closely related to the word neuter and originally meant the same thing: neither male nor female. The meaning of the word changed over the years, as meanings of words often do, until it came to mean not taking sides in a dispute.
Delaney's novel is set during World War II in Ireland, which struggled to remain neutral, fearing invasion and occupation by either England or Germany if it joined the war. The plot involves a man who is married to a missing woman and whose best friend is another woman, who wants his help to catch the man she loves. Yet our hero secretly loves his friend and, like his country, is torn by neutrality.
Slut: Here's another word that means something different today than it once did. John McWhorter writes in What Language Is that slut was originally a word applied equally to persons of both sexes and that it referred to someone who was messy, not sexually promiscuous. Chaucer, for example, asks, "Why is the lord so sluttish, I thee preye?" In the 17th century, McWhorter writes, the word came to mean "high-spirited" and could actually be a compliment. Samuel Pepys once wrote, "Our little girl Susan is a most admirable slut, and pleases us mightily."
Today the word usually refers to a woman's sexual behavior, although there doesn't seem to be any clear definition of how much sex with how many men it takes to become a slut. The word's original meaning is still echoed in phrases like "dresses like a slut." There have been some recent attempts to reform the word, if not the women, and turn it into something positive.