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

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "desuetude":
   abandonment, absentation, abstention, abstinence, casting away,
   cease, cessation, close, closing, closure, conclusion,
   discontinuance, discontinuation, disuse, end, ending, evacuation,
   forsaking, jettison, jettisoning, leaving, nonemployment,
   nonprevalence, nonuse, obsolescence, obsoleteness, obsoletion,
   obsoletism, pensioning off, pulling out, retirement,
   superannuation, throwing overboard, unprevalence, withdrawal



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DESUETUDE. This term is applied to laws which have become obsolete. (q.v.)