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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
In argumentation, an ad hoc argument is one that is hastily constructed to support or explain something without any underlying sense or logical framework. Because of this haste and lack of a consistent frame-work, the explanation is likely to contradict existing thought or other arguments. Usually it happens if someone is put on the spot to explain something - they can either deal with it in a consistent manner (meaning that their arguments are consistent for all eventualities so far), change their consistent beliefs to match, or produce an ad hoc explanation off-the-cuff to dismiss it. Many creationists or 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 nothing more than placebos, but believers will make up excuses as to why the controlled and properly conducted experiment was wrong. 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) you can't do a "double-blind" test on homeopathy. Knowing this, make sure any ad hoc hypothesis
The constant creation of new ad hoc arguments to undermine evidence is a good sign that they are not arguing in good faith - and may lead them into some serious argumentative trouble when their newer ad hoc explanations begin to conflict with others. Creationist explanations for why the Grand Canyon is explained by the global flood but similar canyons aren't seen everywhere are hilariously varied and entirely ad hoc - as no consistent geological theory would posit that a flood would create a canyon, and a flat desert, and a mountain range in different random places.
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|