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]

WordNet (r) 3.0 (2006):

nonentity
    n 1: the state of not existing [syn: nonexistence,
         nonentity] [ant: being, beingness, existence]
    2: a person of no influence [syn: cipher, cypher, nobody,
       nonentity]