The wheels of justice move slowly sometimes, but not, apparently, as slowly as Webster’s New World Dictionary.

Slang has always been a challenge for the courts in cases that involve vulgar or insulting language. Conventional dictionaries lag the spoken word by design. That has lawyers and judges turning to a more fluid source of definitions: Urban Dictionary, a crowdsourced collection of slang words on the Internet.

The online site, created by a college freshman in 1999, has found itself in the thick of cases involving everything from sexual harassment to armed robbery to requests for personalized license plates, as courts look to discern meaning and intent in the modern lexicon.

Last month, Urban Dictionary was cited in a financial restitution case in Wisconsin, where an appeals court was reviewing the term "jack" because a convicted robber and his companion had referred to themselves as the "jack boys."

The court noted that according to Urban Dictionary, "jack" means "to steal, or take from an unsuspecting person or store." It rejected the convicted man’s claim that he should not have to make restitution to the owner of a van he stole to use in a robbery.

Two weeks earlier, a court in Tennessee noted that a phrase used by a manager at a supply chain logistics company — "to nut" — was defined by Urban Dictionary as "to ejaculate." After weighing that and other evidence, it rejected a motion to dismiss a sexual harassment claim by female employees.

It can take years for slang terms to be included in traditional dictionaries, whose editors want to be certain that the words have staying power. By contrast, some new words rush into Urban Dictionary in less than a day. As a result, the site has cropped up in dozens of court cases in recent years, according to a Lexis database of federal and state cases, although the outcome rarely rests solely on a definition.

This trend is likely to accelerate, according to Greg Lastowka, a professor of law at Rutgers specializing in Internet and property law. "If it is Urban Dictionary or hire some linguistic expert to do a survey, it seems like a pretty cheap, pretty good alternative for the court," he said.

In the last year alone, the website was used by courts to define iron ("handgun"); catfishing ("the phenomenon of Internet predators that fabricate online identities)"; dap ("the knocking of fists together as a greeting, or form of respect"); and grenade ("the solitary ugly girl always found with a group of hotties").

Reference in legal cases to Urban Dictionary and Wikipedia, the online encyclopedia, have become common enough that in its Spring 2010 issue, the law review of St. John’s University in Queens published an article that tried to create standardized rules for the most appropriate uses of crowdsourced websites.

Scientific terms and other technical definitions should not be culled from such sites, the article concluded, but it added, "The wisdom of the crowd is an appropriate and valuable reference when consensus itself is at issue, the information is generally known or the content is easily verifiable."

The idea that consensus rules has its skeptics. Tom Dalzell, senior editor of The New Partridge Dictionary of Slang and Unconventional English, is a fan of Urban Dictionary, but he argues that the site has obvious limits.

"Using them in court is a terrible idea; they don’t claim to be an authority or a reference," he said. "Some of the stuff on their site is very good, but there is more chaff than wheat. It is a lazy person’s resource."

Urban Dictionary’s move into the legal arena surprises no one more than Aaron Peckham, its founder, who has continued to run it like a homegrown business. Peckham, who is 32 and lives in San Francisco, has never taken venture capital money and still runs the entire site from his laptop. For revenue, he contracts with others to put advertising on the site and to make merchandise — like T-shirts and mugs printed with some of the site’s more interesting definitions. He has no paid staff members, though he does contract for help with things like advertising and design.

Still, he argues, the development of Urban Dictionary into a tool for courts is "logical."

When he began the site in 1999 at California Polytechnic State University, it was meant to be a parody. "Friends and I would sit around and make up words," he said. As the Internet grew in size, however, contributors from around the globe began to join in and enforce a kind of democratic evaluation of the words.

Urban Dictionary currently gets 110 million monthly page views and is the 77th biggest website in the country, according to Quantcast, a Web analytics company.

Roughly 2.3 million definitions posted on the site — some crude or insensitive — and about 30,000 proposed new definitions are sent in each month, Peckham said. For one to be added, at least five other site members must vote for it — from roughly 7,000 users a month who click "publish" or "don’t publish." Despite the low threshold, some two-thirds of proposals are rejected, Peckham estimates.

Many words are just ignored. Some become popular or disputed. Users can contribute their own definitions to existing words. The word "emo," for instance, currently has more than 1,100 definitions — including "Like a Goth, only much less dark and much more Harry Potter," (definition No. 3) and "sensitive music and the kids that cry while listening to it" (No. 1,122).

The definitions are ranked by popularity, with the idea that democracy will reveal some truth about how the word is really used. "Readers can tell not to put too much faith in a definition that is really unpopular," Peckham said.

He added: "Dictionaries may be more heavily researched, but the real authority on language and the meaning comes from people who speak the language. The whole point of Urban Dictionary is we are defining our own language as we speak it."

Jesse Sheidlower, editor at large for the Oxford English Dictionary, points out, however, that popular does not mean accurate. "People may like a word because it was posted by their friend or because it was funny," he said. (Peckham said that private analyses the site has conducted show that "funny" is the No. 1 reason people give for voting for posts.)

Sometimes a common slang meaning has nothing to do with what a person actually meant. In 2009, the Nevada Supreme Court said that the Department of Motor Vehicles could not deny the personalized license plate "HOE" because Urban Dictionary said it meant prostitute. "A reasonable mind would not accept the Urban Dictionary entries alone as adequate to support a conclusion that the word ‘HOE’ is offensive or inappropriate," the justices wrote.

In fact, William Junge said that he wanted the plate for his Chevrolet Tahoe only because "TAHOE" was not available. Junge, who was 62 at the time, said the idea that it could mean whores had not crossed his mind.

As he told reporters at the time: "That was their interpretation. Shame on them."