Or, if you prefer, it may not surprise you to find out that however clever you are, if you are well-to-do, the language of the street may be incomprehensible to you. Which might make you look very unfashionable, but at least explains the mystery.

Some language, of course, transcends all social groupings - in particular the sorts of words that lewd 11-year-old boys search diligently through dictionaries for.

For those boys, D-Day has arrived in the form of Jonathan Green's new Dictionary of Slang. It is, publishers Cassell claim, the only dictionary to include 997 words for penis, 856 for vagina, 1,232 for sexual intercourse, and 797 for masturbation.

With a strike rate like that, no hard searching for rude words is needed. In fact, Green says: "In my book the challenge is to find a page without one on."

The book has taken Green five-and-a-half years to write, but is based on his collection of slang words going back 20 years.

Counterblast

For him the subject is interesting because it is the language in use on the street, rather than the language of conventional dictionaries.

"Slang is the street," he said. "We know that the Romans and the Greeks had it, I have no doubt whatsoever that other civilisations had it too.

"My definition of slang is that it's a counter language. It's oppositional, it's youth, it's urban and it's street. It's exciting."

The immense task of cataloguing more than 75,000 words is made harder by the way slang is so culturally specific. Many cultures are reflected in the book, including American, Australian, black, gay, rap and the world of drugs.

But it doesn't stop there, for it also reflects slang down the centuries since the first collection appeared in 1531.

Pedigree chum

"With standard English," he said, "you have got most of the words you need to say what you want to say. But they are quite old words, and on the whole in standard English, we don't come up with new words for our emotions. The new ones tend to be technical, scientific, or computer related." Which is where the street language comes in.

But just because many of the words polite conversation uses are old, it does not mean slang has no pedigree. There is some truth, Green says, when people call them "good Anglo-Saxon words".

The word "arse" and its American equivalent "ass" , for instance. They stem from the word "ars", first recorded as being translated from the Latin "podex" by Abbot Ælfric who lived in England at the turn of the last millennium.

He also translated the Latin "testiculi" for testicles into "beallucas".

Have some respect

These and other words were part of the respectable English vocabulary until the late 17th and early 18th centuries. However, Green said, in the age of Dr Johnson when those who spoke correct English wanted to appear smart and respectable, many such words became unacceptable.

So in 1796, the lexicographer Grose would not print many of them. The common term for vagina he printed as "c - tt", and defined as "a nasty word for a nasty thing".

This process continued well into this century. Eric Partridge, for decades the authority on slang, would not even use the word "penis" in his 1937 dictionary, preferring the term "membrum virile".

Although expression is much freer today, even now, BBC guidelines insist that certain four-letter words should not be used "without advance reference to and approval from channel and network controllers".

The use of bad language in the media remains one of viewers' and listeners' prime concerns, despite its increased currency in respectable contexts. But it is not the only thing that has changed.

Slurring speech

Green said although Partridge had not included the so-called "dirty" words, racially offensive words such as "kike", "nigger", and "wog" appeared without any warning at all that they were derogatory terms.

Green's dictionary includes warnings of racial slurs. By doing this, and by not being squeamish about using explicit language, he is doing no more than doing what slang itself does - reflecting people's values as they are today.