The Collaborative International Dictionary of English v.0.48:

Unsociable \Un*so"cia*ble\, a.
   Not sociable; not inclined to society; averse to
   companionship or conversation; solitary; reserved; as, an
   unsociable person or temper. -- Un*so"cia*ble*ness, n. --
   Un*so"cia*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unsociableness
    n 1: an unsociable disposition; avoiding friendship or
         companionship [syn: unsociability, unsociableness]
         [ant: sociability, sociableness]