The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

maiden aunt
    n 1: an unmarried aunt