noun
Familiar, somewhat old-fashioned term used to address or refer to a young woman, especially a maid or a girl from a good family. Often used by older people or in historical settings.
お屋敷のねえやが庭で花を摘んでいた。
The young lady of the mansion was picking flowers in the garden.
「ねえや、ちょっとお茶
The old woman said, "Young lady, could you make some tea?"
お嬢さん is the standard polite term for a young lady or someone's daughter, while ねえや is a more familiar, old-fashioned, and somewhat rustic way to address a young woman, often a maid or a girl from a traditional household.
Derived from 姉 (ane, 'older sister') with the familiar suffix や, used as a term of endearment or respect for a young woman. The exact historical development is uncertain.