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Ad hoc means thrown together (literally, to this). In modern usage, this phrase has two fairly distinct meanings, depending on whether it is being used in argumentation or government.
 Legitimate and common uses
Ad hoc explanations are not always an illegitimate exercise; if a new phenomenon is discovered, early explanations are likely to be technically ad hoc until experimentation or study can be conducted on it. In this sort of use and more general use, ad hoc arrangements are unavoidable because no one can predict the unpredictable (except maybe Derren Brown) - e.g., examiners putting ad hoc arrangements on exam marks because they didn't predict a spelling error, or role-players making up rules as they go along because the source book never said what to do if a Level 12 Sorceress cast that spell from that location while this rule was in effect under those rules in that building, which just produces a silly result. In science or when investigating pseudoscience, it is important to make sure that ad hoc claims are falsifiable and repeatable.
 In argumentation
According to many sources (go look), an ad hoc argument is one that is something added to an argument or hypothesis only after the hypothesis is exposed to be in need of saving.
When a hypothesis is in such trouble and every problem posed to it (or to it’s defender) is always met with a new item that seems to be an excuse, people tend to recognize something is suspicious. Their inability to articulate what is suspicious is what probably gives rise to this rather weak and useless definition of “ad hoc”. That is, useless as far as telling you when the argument or hypothesis has an actual problem (that makes it unlikely to be true), or help you decide which hypothesis is more likely.
What such people are probably trying to articulate is something more like the following: items in a hypothesis which are not supported by evidence to be true make that hypothesis also unlikely to some degree. Thus, adding more unsupported items does not save a hypothesis, but makes it even less likely to be true. This usage is also similar to that stated in the Oxford Reference.
Anyways, many creationists and woo pushers use ad hoc arguments to explain away evidence that contradicts their underlying beliefs, rather than revising those beliefs. For example, many alternative medicines have been disproved or shown to be merely placebos, but believers will make up excuses as to why the controlled and properly conducted experiment was wrong. Some homeopaths, for instance, will cry that the succussion process was carried out incorrectly (as if 9 bangs rather than 10 makes all the difference), or that (inexplicably) it is impossible to do a "double-blind" test on homeopathy. Creationist explanations for how the Grand Canyon is explained via the global flood while similar canyons aren't seen everywhere are hilariously varied and entirely ad hoc.
In politics, ad hoc refers to the creation of "temporary" committees or processes to handle new situations - similar to the valid uses of ad hoc arrangements described above, the situations are new, so can't really be dealt with in any other way. In some cases, notably due to laziness, these temporary creations end up becoming permanent. The constant "ad hoc tinkering" often creates solutions that work well in the short term, but end up creating a system that no one would have intentionally devised, given the time to work it out properly at the beginning. However, due to path dependency, it's basically impossible to go back and redesign the whole system from scratch. Instead, people make small ad hoc changes to the system again, and the process continues, with each new implementation causing a sort of partially-directed evolution of government.
 See also
| Articles about logical fallacies|
|Affirming the consequent||Denying the antecedent|
|Fallacious argument styles|