Source : Webster's Revised Unabridged Dictionary (1913)

Goody-goody \Good"y-good`y\, a.
   Mawkishly or weakly good; exhibiting goodness with silliness.
   [Colloq.]

Source : WordNet®

goody-goody
     adj : affectedly or smugly good or self-righteous
     n : a person who behaves extremely well in order to please a
         superior