The Collaborative International Dictionary of English v.0.48:

nincompoop \nin"com*poop\, n. [A corruption of non compos.]
   A fool; a silly or stupid person. [Law]
   [1913 Webster]

         An old ninnyhammer, a dotard, a nincompoop, is the best
         language she can afford me.              --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "nincompoop":
   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, ninny, ninnyhammer, nitwit, noddy, put, stupid, thickwit,
   tomfool, witling