“I used to ask my mother, ‘Why Glom?’ and she’d say, ‘Because you’re always grabbing, always taking things,’” he said, imitating his mother’s accent and limited patience, shaped by a lifetime in Irish neighborhoods in New York City.

It was not exactly an etymological explanation, and Mr. Cassidy’s curiosity about the working-class Irish vernacular he grew up with kept growing. Some years back, leafing through a pocket Gaelic dictionary, he began looking for phonetic equivalents of the terms, which English dictionaries described as having “unknown origin.”

“Glom” seemed to come from the Irish word “glam,” meaning to grab or to snatch. He found the word “balbhán,” meaning a silent person, and he surmised that it was why his quiet grandfather was called the similarly pronounced Boliver.

He began finding one word after another that seemed to derive from the strain of Gaelic spoken in Ireland, known as Irish. The word “gimmick” seemed to come from “camag,” meaning trick or deceit, or a hook or crooked stick.

Could “scam” have derived from the expression “’S cam é,” meaning a trick or a deception? Similarly, “slum” seemed similar to an expression meaning “It is poverty.” “Dork” resembled “dorc,” which Mr. Cassidy’s dictionary called “a small lumpish person.” As for “twerp,” the Irish word for dwarf is “duirb.”

Mr. Cassidy, 63, began compiling a lexicon of hundreds of Irish-inspired slang words and recently published them in a book called “How the Irish Invented Slang,” which last month won the 2007 American Book Award for nonfiction, and which he is in New York this week promoting.

“The whole project started with a hunch — hunch, from the Irish word ‘aithint,’ meaning recognition or perception,” the verbose Mr. Cassidy said in an interview on Monday at O’Lunney’s, a bar and restaurant on West 45th Street. He has worked as a merchant seaman, a labor organizer and a screenwriter, and he lives in San Francisco, where he teaches Irish studies at the New College of California.

He pulled out his pocket Irish dictionary and began pointing out words that he said had been Americanized by the millions of Irish immigrants who turned New York into an extension of the Ghaeltacht, or Irish-speaking regions of Ireland.

“Even growing up around it, little shards of the language stayed alive in our mouths and came out as slang,” he said, spouting a string of words that sounded straight out of a James Cagney movie.

“Snazzy” comes from “snasach,” which means polished, glossy or elegant. The word “scram” comes from “scaraim,” meaning “I get away.” The word “swell” comes from “sóúil,” meaning luxurious, rich and prosperous, and “sucker” comes from “sách úr,” or, loosely, fat cat.

There is “Say uncle!” (“anacal” means mercy), “razzmatazz,” and “malarkey,” and even expressions like “gee whiz” and “holy cow” and “holy mackerel” are Anglicized versions of Irish expressions, he said. So are “doozy,” “hokum,” “humdinger,” “jerk,” “punk,” “swanky,” “grifter,” “bailiwick,” “sap,” “mug,” “wallop,” “helter-skelter,” “shack,” “shanty,” “slob,” “slacker” and “knack.”

Mr. Cassidy chatted with an Irish-born worker at O’Lunney’s, Ronan O’Reilly, 21, who said he grew up in County Meath speaking Irish. He nodded in agreement as Mr. Cassidy explained that Irish survived in New York as slang.

“It was a back-room language, whispered in kitchens and spoken in the saloons,” Mr. Cassidy said.

Mr. O’Reilly nodded and said, “Sometimes my friends and I will use it amongst ourselves, sort of like an underground language.

“Some of your words here sound like they are taken straight from Irish, even expressions directly translated, like ‘top of the morning’ or ‘thanks a million,’” he continued. “In Ireland, we pick up American slang from TV, like the word ‘buddy.’”

Mr. Cassidy laughed. “Buddy,” he contends, actually comes from “bodach,” Irish for a strong, lusty youth.

Another employee came up, Lawrence Rapp, 25, who said he was an Irishman from London, where the art of rhyming slang is practiced.

“If you have to piddle, you say ‘Jimmy Riddle,’” he said.

Mr. Rapp said Londoners often used the word “geezer” to describe people, and Mr. Cassidy pointed out that the term derives from the Irish word “gaosmhar,” or wise person.

“Even the word ‘dude’ comes from the Irish word ‘dúid,’ or a foolish-looking fellow, a dolt,” Mr. Cassidy said. “They called the guys dudes who came down to the Five Points section of Manhattan to chase the colleens.”

He showed a passage in his book that notes that the Feb. 25, 1883, edition of The Brooklyn Daily Eagle reported the coining of the word “dude,” referring to, among other things, a man who “wears trousers of extreme tightness.”

“You dig?” he said. “‘Dig,’ as in ‘tuig,’ or understand.”