The Collaborative International Dictionary of English v.0.48:

Nonentity \Non*en"ti*ty\, n.; pl. Nonentities.
   1. Nonexistence; the negation of being.
      [1913 Webster]

   2. A thing not existing. --South.
      [1913 Webster]

   3. A person or thing of little or no account; an unimportant
      person; -- usually used as a disparaging term. [Colloq.]
      [1913 Webster +PJC]