Source : Webster's Revised Unabridged Dictionary (1913)

Nincompoop \Nin"com*poop\, n. [A corruption of non compos.]
   A fool; a silly or stupid person. [Law]

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

Source : WordNet®

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