The Collaborative International Dictionary of English v.0.48:

Desuetude \Des"ue*tude\, n. [L. desuetudo, from desuescere, to
   grow out of use, disuse; de + suescere to become used or
   accustomed: cf. F. d['e]su['e]tude. See Custom.]
   The cessation of use; disuse; discontinuance of practice,
   custom, or fashion.
   [1913 Webster]

         The desuetude abrogated the law, which, before, custom
         had established.                         --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

desuetude
    n 1: a state of inactivity or disuse