A good dictionary is not a book of rules. It's an up-to-the-minute history of the past and present usage of a language.

The 5,000 new words in the magnificent new 20-volume second edition of the Oxford English Dictionary include many that affront the genteel and irritate the persnickety.To "bonk" is in. First used in print in 1931, it was once lower-class British slang for punching someone on the nose. In the early 1970s it was gentrified to mean having sexual intercourse. In the 1980s it has been vulgarized by the British tabloids.

"Yuppification" is there, too, somewhat blurred during its crossing of the Atlantic. In New York a yuppie is, or was originally, a member of a definable category of young urban professionals with sufficient disposable income to make them an ad man's target group. In Britain, the word is used more broadly as a term of denigration of any middle-class individual, from a farmer's son to a suburban secretary, guilty of a distinctive style of behavior judged to be affected - just another weapon in the armory of the English class war.

Old-fashioned Britons who believe Americans are messing up the language will find plenty of ammunition. "Bright-eyed and bushy-tailed" is included - here, as in America, drained of its original color by overuse. So are "street cred," "scam," "bazoom" (meaning women's breasts), "zap," "the whole caboodle," "diet cola," "Rambo," "the pits" and "rap."

Imported U.S. films and TV series continue to exercise a significant influence on English speech - though those of us who enjoy using American slang in Britain are, to be honest, usually a few years out of date.

The irritation factor in the new OED is predictably high. Neologisms like "chair" (meaning chairman or chairwoman) and "ageism," invented to make a political point, are rife. So are ignorant misuses of words, known to lexicographers as "popular etymology," that make pedants go red in the face but which, once they have crept into the language, are ineradicable - and which, a generation later, become sanctioned by the educated.

Current examples are: "disinterested" (in its incorrect usage as "uninterested" as well as its correct one as "unbiased by personal interest"); "hopefully" (meaning "it is hoped that"); and "jejune" (meaning "naive" instead of "meager").

Part of the fascination of dictionaries is the discovery that words and phrases that sound unmistakeably modern have actually been around for a long time. Who would have guessed that "for love or money" dates from 971 AD? Or the English vogue use of "angst" from 1849 (the novelist George Eliot)? Or "acid rain" from 1859? Or "instant coffee" from 1915?

Some new definitions are so parochial that one wonders what non-British - indeed, non-London - users of the OED will possibly make of them. One would rather teach an American the rules of cricket than try to explain to him the complex recent etymology of "Ugandan affairs" (a coterie phrase for "sexual intercourse") or "tired and emotional" (meaning "drunk') - both evolved from overworked jokes in the satirical British magazine Private Eye.

The OED is customarily described as "a great British institution." So it is. But nowadays it is also a great global institution. Far more people born outside the British Isles speak English than natives.

Perhaps we can claim the OED as the last and greatest instrument of benevolent British imperialism.