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]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "nonentity":
   Middle America, Milquetoast, a nobody, a nothing, absence, baby,
   big baby, bourgeoisie, burgherdom, chicken, cipher, common man,
   crybaby, deprivation, doormat, dud, dull tool, dummy, emptiness,
   figurehead, fourth-rater, gutless wonder, hollow man,
   insignificancy, invertebrate, jackstraw, jellyfish, lay figure,
   lightweight, little fellow, little guy, man of straw, mediocrity,
   meek soul, middle class, middle order, middle orders, milksop,
   mollycoddle, namby-pamby, nebbish, negation, negativeness,
   negativity, nihility, no great shakes, no prize, nobody,
   nobody one knows, nobody special, nonbeing, nonexistence,
   nonoccurrence, nonreality, nonsubsistence, not-being, nothing,
   nothing special, nothingness, nullity, obscurity, pansy,
   pantywaist, pip-squeak, punk, puppet, pushover, runt, rushlight,
   sad sack, scrub, second-rater, shrimp, silent majority, sissy,
   small, small beer, small fry, small potato, small potatoes,
   softling, softy, sop, squirt, squit, suburbia, thing of naught,
   third-rater, tinhorn, trifle, unactuality, unreality, vacancy,
   vacuity, vacuum, void, weak sister, weakling, whiffet,
   whippersnapper, zero, zilch