A historical
doctrine, principally applicable to
criminal law, that prohibited enforcement
of a law which has fallen into disuse.
The idea is that authorities give notice of a law to
citizens by enforcing it;
lack of enforcement (for a commonly broken law, especially) indicates an implicit
repeal.
Although the doctrine is common in most European civil law nations and a practical
necessity in poorly formalized common law jurisprudences, it has been rejected
in most jurisdictions in the U.K. and the U.S. Many rejections came in the first
half of the twentieth century, but defenses are still sometimes mounted on this basis:
In Commonwealth v. Stowell (1983), a Massachusetts court upheld a conviction
of adultery. Stowell argued that adultery had not been prosecuted in decades, and therefore, under desuetude, she could not be convicted. The court rejected her defense
and the legal validity of desuetude.
Even where it has been officially rejected, many vestiges remain, such as:
Some information from Peter Suber, "The Paradox of Self-Amendment:
A Study of Law, Logic, Omnipotence, and Change"
(http://www.earlham.edu/~peters/writing/psa/index.htm).