Up until the sixteenth century, the English word for girl was used to refer to a youngster of either the male or female gender. Trying to relate the story of some rebellious child in medieval times might have proven difficult. In order to refer to a specific sex someone had to add a qualifier.
For a boy this qualifier was knave. An author during those times might write, "The knave girl stepped our of the barn," if he was narrating a story of a boy in a barn.
Girls were referred to as gay and might be addressed as such, "the gay girl fixed her hair in the morning." Of course, these aren't the only examples of these interchangeable words.
"Man" and "men" were also used as gender neutral terms when they were originally formed. Man was used to refer to any "thinker" or member of the human race and the word "men" meant "to think" or "to have a cognitive mind." The difference implemented nowadays to help decipher between man, meaning a male, and man, meaning a thinker, is a simple capital "M."
