A rule of thumb is an informal principle that is intended to provide general guidance rather than precise direction.
People once used the phrase rule of thumb in reference to a rough physical measurement, but the term has evolved to apply to any kind of broadly applicable and imprecise rule. It's not expected that a rule of thumb will always provide the correct answer.
For example, see below three rules of thumb for dealing with Internet problems:
1. If you get a message saying the domain name server (DNS) can't find your page and you're sure you've typed it in correctly or clicked on a valid link, try it again -- twice. Sometimes packets don't get where they're supposed to.
2. If you get a 404 error (page not found) message, the page may be temporarily missing because of some issue at the target site. Try it again later or send an email informing the webmaster of the problem and asking them to restore the page.
3. Be aware that sometimes a page you've visited recently may be coming from your cache or the cache on a proxy server within your company. To get the "fresh" version of the page, click on reload in the menu.
Rules of thumb often yield best practices. For example, in intrusion detection, a rule of thumb is that records and profiles should have unique identifiers, insofar as possible. That rule of thumb yields the best practice of including full path names for files and following a user identification scheme that binds a user ID to the system on which it resides.
The origin of the phrase rule of thumb is somewhat contentious. Many claim that the original "rule of thumb" was legislation that at one time made it legal for a man to beat his wife, although not with any implement thicker than his thumb. References to such a law exist; however, no documentation of the actual law has ever been located.
The reference to a rough measurement for carpentry and agricultural applications is a more likely source of the phrase. The earliest documented use is in this sense is in J. Durham’s Heaven upon Earth (1685): "Many profest [professed] Christians are like to foolish builders, who build by guess, and by rule of thumb."