The Collaborative International Dictionary of English v.0.48:

Ninny \Nin"ny\, n.; pl. Ninnies. [Cf. It. ninno, ninna, a
   baby, Sp. ni[~n]o, ni[~n]a, child, infant, It. ninna, ninna
   nanna, lullably, prob. fr. ni, na, as used in singing a child
   to sleep.]
   A fool; a simpleton. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ninny
    n 1: a stupid foolish person [syn: nincompoop, poop,
         ninny]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "ninny":
   Boeotian, ass, block, clod, dimwit, dolt, donkey, dope, dullard,
   dumb cluck, dumbbell, dummy, dunce, gowk, idiot, imbecile, jackass,
   jerk, jobbernowl, lackwit, lamebrain, lightweight, looby, loon,
   niais, nincompoop, ninnyhammer, nitwit, noddy, put, stupid,
   thickwit, tomfool, witling