unpaired

from WordNet (r) 3.0 (2006)

unpaired
    adj 1: of the remaining member of a pair, of socks e.g. [syn:
           {odd}, {unmatched}, {unmated}, {unpaired}]

from The Collaborative International Dictionary of English v.0.48

Unpaired \Un*paired\, a.
   Not paired; not suited or matched.
   [1913 Webster]

         And minds unpaired had better think alone. --Crabbe.
   [1913 Webster]