'maiden aunt' definitions:

Definition of 'maiden aunt'

(from WordNet)
noun
An unmarried aunt

Definition of 'maiden aunt'

From: GCIDE
  • 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. [PJC]