Maid was a customary title in the Scottish Highlands for the heiress to a chiefship or chieftainship.
Once they take on the lairdship in their own right, they will then become styled as "lady" and the title of "Maid of [x]" will pass onto their eldest daughter.
If a laird has a son who is the heir apparent but still has an elder daughter, she is still entitled to become styled as "maid".
If the eldest daughter of a laird chooses to accept the title it would be styled as "Miss [Forename] [Surname], Maid of [Lairdship]" (e.g. Miss Jane Smith, Maid of Edinburgh).
If a laird has any younger daughters, they are styled as "Miss [Forename] [Surname] of [Lairdship]"