Do you know your “tum-tum” from your “ching” and your “corn” from your “gwop” (gun, knife, ammunition and money)? Neither do police and prosecutors, who have begun consulting a linguistics professor to help decipher urban slang and drill lyrics used as evidence in criminal investigations.

The complexity of inner-city dialects and the growing use of texts and social media posts in court evidence has forced detectives and lawyers in London, the West Midlands and Essex to seek translations, according to Tony Thorne, an academic at King’s College London, who has been studying youth slang since 1990.

Thorne has compiled dictionaries of hundreds of slang words and a vocabulary of drill, a form of rap music which often deals with real-life violence. He said he has advised police on more than a dozen cases, including one where police believed the use of the word “plug” meant stab, but it was being used to describe a source.

The dialect has become known among academics as multi-ethnic London English (MLE), though is not limited to the capital. Last autumn, an image circulated of a glossary of “youth language” on a whiteboard in a Lancashire police station including “peng = attractive, feds = police, swear down = tell the truth”.

Courts in places such as Northampton are also struggling to deal with its shifting meanings; schoolchildren in east Yorkshire are speaking the dialect, as it spreads rapidly through song lyrics and the internet.

“I am advising defence lawyers, criminal prosecutors and police with interpreting and translating language which is being used in evidence,” Thorne said. “If they want to dispute evidence they need someone like me to translate. They put me on the list with translators of Hindi and Gujarati.”

Thorne has a network of informants including teachers, youth workers and grime and drill enthusiasts to help him, but admits there are holes in his knowledge and that it might seem anomalous that an “elderly white guy” would seek to be an expert in the dialect.

One former drug-dealer in west London told the Guardian of several words for drugs he has not yet listed, including yay for crack cocaine, pebs for heroin and amm, cro and tweed for cannabis.

Femi, 18, a drill producer from Leytonstone in east London, said: “If it was a young person like me translating it would be more accurate. You want to understand the context. You have to be using the language, listening to drill and the comments people post beneath songs on YouTube.”

Thorne said he was “not posing as an insider on black and minority ethnic street culture but I am someone who has a linguistic perspective that people on the street aren’t likely to have”.

He added: “I am trying to help by defending kids who are wrongly accused by their language and go after the people who have committed violent crimes.”

Thorne is typically handed transcripts of drill lyrics and phone messages to decipher, most recently a lyric about a knife attack found on a suspect’s phone.

MLE mixes white working-class English with patois, largely from black Caribbean dialect, but with some Arabic and Polish. It is rich in status words (badmanz – tough male, bozz – leader, wallad – foolish male) and relationship words (darg – attractive male, game – flirtatious, begfriend – sycophant, fam – group).

The most obscure words tend to be about weapons and drugs. Stab can be plug, ching, bore, dip, kweng, splash. Gun has even more options: burner, wap, hand ting, iron, leng, mash, mac, mop, scram, skeng, spinner, stick, trey, tum-tum and wap. Words for knife include skeng, ox, Rambo, ramsey, shank and sword.

“It shows that, tragically, weapons are a really important symbolic part of their identity,” said Thorne. It also shows how MLE acts as a “cryptolect” – a language meant to hide things. It is also an example of “hypersynonymy” similar to how Eskimos have many words for different types of snow, Thorne said.

MLE is attracting growing interest and Jeffrey Boakye, a writer and headteacher in south Yorkshire who is originally from Brixton in south London, addresses it in a forthcoming book on black British culture, Black, Listed.

“One simple theory is that MLE is taking root because of its indelible association with Jamaican patois,” he writes. “The fact remains that the Caribbean influence has been cool for generations ... MLE might just be a cool way of speaking.”

He added: “Kids round here in east Yorkshire know all the cool London language that I knew when I grew up.”

Jeannie Mackie, a defence barrister, said police often testify that they understand the meaning of MLE found on defendants’ phones or in drill lyrics, but in fact “slang changes all the time and is a fast-moving linguistic flow”.

She said: “Where it’s becoming really serious is where prosecutors are looking at lyrics. Police look through social media, particularly Facebook and YouTube, and find scary lyrics and attribute that to being a personal expression of a defendant and what they did.”

She said the need for translators showed “the vast gaps between sections of society”.

A Met police spokesperson said: “We are constantly in discussion with a number of experts to see how we can develop and expand our expertise.”

Lawrence Henderson, a defence barrister operating mostly in Northampton, said: “When you have a patois-speaking defendant and a white officer interpreting, it is fraught with danger. The guys dealing drugs are making sure the language is slipping and sliding all the time, so the message is obvious to the person receiving it but not a copper.”

Thorne said MLE was probably the most fully developed dialect, or sociolect, since Polari, which died out in the 1970s. A blend of Italian, Occitan, French and other sources, it was used by homosexual, theatrical and circus communities and has passed words such as naff (dull), khazi (toilet) and cottaging (seeking sex in public conveniences) into modern English. Other vocabulary like corybungus (backside) and orderly daughters (police) are all but lost.

The MLE word “paigon” caused mainstream head-scratching when it was used by the rapper Stormzy to describe Theresa May when he was picking up an award in 2017. It didn’t mean non-religious. Thorne translates it as “untrustworthy person, enemy”.

“[MLE] has a social and cultural power and is evolving in a way most slangs aren’t,” he said. “It points up the real diversity of Britain and it is not ghettoised ethnicity. The theorists call it super-diversity.”