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]