Sod's law is a name for the axiom that "if something can go wrong, it will", with the further addendum, in British culture, borrowed from Finagle's law, that it will happen at "the worst possible time". This may simply be construed, again in British culture, as "hope for the best, expect the worst".
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The phrase is seemingly derived, at least in part, from the colloquialism an "unlucky sod"; a term for someone who has had some bad unlucky experience, and is usually used as a sympathetic reference to the person
The term is still used in the United Kingdom, though in North America the eponymous "Murphy's law" is more popular.
Comparison with Murphy's law
Sod's law is similar to, but broader than, Murphy's law ("Whatever can go wrong will go wrong"). For example, concepts such as "bad fortune will be tailored to the individual" and "good fortune will occur in spite of the individual's actions" are sometimes given as examples of Sod's law in action. This would broaden Sod's law to a general sense of being "mocked by fate". In these aspects it is similar to some definitions of irony, particularly the irony of fate. Murphy's technological origin on John Stapp's Project MX981 is more upbeat—it was a reminder to the engineers and team members to be cautious and make sure everything was accounted for, to let no stone be left unturned—not an acceptance of an uncaring uninfluenceable fate.
According to David J. Hand, emeritus professor of mathematics and senior research investigator at Imperial College London, Sod's law is a more extreme version of Murphy's law. While Murphy's law says that anything that can go wrong, will go wrong (eventually), Sod's law requires that it always goes wrong with the worst possible outcome. Hand suggests that belief in Sod's law is a combination of the law of truly large numbers and the psychological effect of the law of selection. The former says we should expect things to go wrong now and then, and the latter says we remember the exceptional events where something went wrong, but the great number of mundane events where nothing exceptional happened are forgotten.
Examples
Both Dawkins and Hand offered these examples as part of a critique of the thinking that leads to belief in such laws, rather than examples supporting the existence of the law. Both went on to explain the effect in terms of selection bias and the law of truly large numbers.