The word girl made its arrival in the language in the early 1300s. Back then it was gender-neutral and was used to refer to kids of both sexes pretty much like the word child is today. So girls were called girls, and so were boys too!
The origin of girl remains an unsolved mystery, though a number of theories have been put forward in attempts to trace it.
About a hundred years later, at the turn of the 15th century, the word girl began to be used in its current sense. And at about the same time the word boy, which is believed to be a French borrowing, changed its original meaning servant, commoner and even scoundrel to male child.
Amazing, isn’t it?!