[PETER QUINN is a novelist and essayist, and a chronicler of Irish-America. A third-generation New Yorker whose grandparents were born in Ireland, Quinn is the author of Banished Children of Eve (1994), which won the American Book Award. This essay forms the introduction to Daniel Cassidy’s How the Irish Invented Slang. His latest book is Looking for Jimmy: A Search for Irish America.]

In 1799, troops with Napoleon’s army in Egypt unearthed an ancient tablet inscribed with a tribute to the Pharaoh in demotic script as well as Greek and hieroglyphs. As a result of this discovery outside the town of Rashid (Rosetta), the Egyptologist and linguist Jean-Francois Champollion was eventually able to reveal the meanings of a once-indecipherable language. What had been lost was found, and historians and scholars gained a new understanding of the past. Working with a pen (or more likely, a computer) rather than a spade, and serving both as digger and decoder, Daniel Cassidy presents us with revelations that are, for etymologists in general and Irish Americans in particular, every bit as momentous as those Champollion extracted from the Rosetta stone.

The discoveries that Cassidy has gathered into How the Irish Invented Slang: the Secret Language of the Crossroad represent a hugely significant breakthrough in our ability to understand the origins of vital parts of the American vernacular. He has solved the mystery of how, after centuries of intense interaction, a people as verbally agile and inventive as the Irish could seemingly have made almost no impression on English, a fact that H.L. Mencken, among other students of the lan-guage, found baffling. What was missing, it turns out, wasn't a steady penetration of Irish into English, but someone equipped with Cassidy's genius -- a unique combination of street smarts and scholarship, of memory, intuition, and intellect-who could discern and decipher the evidence.

Like the Frenchmen who uncovered the Rosetta stone, Cassidy's discovery began with a serendipitous dig, a solitary stroke of the spade into the fertile earth of his own family's history, at the spot where a piece of the past jutted above the layers of time forgotten or obscured in the form of a single word,"Boliver" (bailbhe, balbhán, mute, inarticulate, a silent person), the semi -affectionate, semi-sarcastic nickname used to refer to his taciturn grandfather. Beginning with that key, a la Champollion, Cassidy unlocks the secret of a centuries-long infiltration of Irish into English, exactly where it would be most expected, amid the playfully subversive, syncretic, open-ended olio of slang."We were not balbh (mute) in Irish," writes Cassidy:

“The slang and accent of five generations and one hundred years in the tenements, working-class neighborhoods, and old breac-Ghaeltachta (Irish-English speaking districts) slums ('s lom, is a bleak exposed place) of Brooklyn and New York City held within it the hard--edged spiel (speal, cutting language) and vivid cant (caint, speech) of a hundred generations and a thousand years in Ireland: Gaeilge, the Irish language.”...