Longman Dictionary of Contemporary English
The Collaborative International Dictionary
maiden aunt \maiden aunt\ n. Literally, an aunt who has never been married. Figuratively, it is a term used as the prototype of a person who is broadly naive and not wise in worldly ways; as, he knows as much about programming as my maiden aunt.
WordNet
n. an unmarried aunt
Usage examples of "maiden aunt".
But Jo had her own eyes to take care of, and feeling that they could not be trusted, she prudently kept them on the little sock she was knitting, like a model maiden aunt.
Might be all right for a dried-up maiden aunt, but definitely not for a pretty lady like yourself.
And if they could get that thing out of the building without knocking down a few walls, he was a Dutchman's maiden aunt.
She looked like somebody's maiden aunt come up out of the grave, I mean as frumpy as a Cow.