Top Definition
nannyboo is a person of either gender who refuses to grow up, not just in one area of his/her life but in every area and in general
Nannyboo boo is something a small child would say while sticking out their tonge and making faces to tease another child. This is where the term nannyboo comes from.

An immature person is not exactly a good comparison as an immature person can become mature later in life. This is a person who has the ability to be mature and chooses not to.
by dra2333 February 09, 2010

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