Update on the avocado question, below.
There have been books and articles lately describing how the Irish gave us much of our slang. The overarching theory is that gamblers, drifters, and underworld figures of the early 19th C were often Irish (the Italians came a bit later), and Gaelic words took on English pronunciations and spellings. These words, such as jazz, honkty-tonk, slum, have similar meanings to Gaelic teas (pronounced tiaz) meaning “heat, enthusiasm,” Aing iht Tarraing meaning “evil lure,” and saol luim meaning “hard world, poor world.”
All very plausible, of course. But plausibility is not the only measure.
Upon hearing words in an unfamiliar language, our brains quite automatically seize on bits that sound like our own language. “Sparrow-grass” was what Mainers heard from the word asparagas. My son showed off knowing tak sa mycket,* Swedish for “many thanks” in first grade. A friend laughed: “Toxic mittens? What kind of a thank you is that?” On our first visit to Romania we were taught to say cu placere, “with pleasure.” One woman nodded. “Couple ‘a cherries. Got it.”
We have words and phrases like this all over English. The Mayan vegetable Ahucatl sounded like Spanish for lawyer, "avocado." Update in response to question, from Your Dictionary: Originally, the Aztecs called this fruit "ahucatl" in their language, Nahuatl, and believed it was an aphrodisiac. To the Spaniards, the Nahuatl word "ahucatl" sounded like their word, avocado "lawyer" (spelled "abogado" today). The first recorded English usage in 1697 was the compound "avogato pear." The Aztecs also made sauces, called "molli" in Nahuatl. That made their avocado sauce, of course, "ahuacamolli," shortened by the Spaniards to "guacamole" [hwah-kê-'mo-le], the popular chip dip today. A briar pipe has nothing to do with wild roses or other thorny bushes, but the root of the bruyere tree of the Mediterranean. The Cree Indians had a word otchek (Ojibwa otchig) for the groundhog. The English made it “woodchuck” and applied it to another animal. Kitty-corner or catty-corner comes from cater, meaning four. (Cf. Quarter, quarto). Happens all the time. It is called a folk etymology by linguists and etymologists.
So it’s not a ridiculous idea that all these slang phrases might be Gaelic in origin. The problem is there’s not a lot of evidence for it. The immediate protestation from Daniel Cassidy and others is that Gaelic was a suppressed language which migrated to the underworld, where it was seldom written down. Whether in London, Liverpool, New York, or New Orleans, the Irish were an underclass, and their language denigrated or even forbidden for many years. Thus, the usual documentary evidence we might hope for is simply not going to be available for Irish Gaelic in the 18th and 19th Centuries. Nonetheless, we have some record of Gaelic before and after, and some unusual reference points such as NY Police Chief George Matsell’s The Secret Language of Crime from 1859. Much of that information supports Cassidy’s claims partially. I know of no instance in which it provides a definite proof.
Linguistics is about proof; etymology is about probability. That’s an unfair simplification, but it will do for now. Cassidy and others who wish to defend Irish honor in the field of contribution to English get quite caught up in the oppression aspect, and recognizing Gaelic roots of terms begins to take on an aspect of “We have to do this to right ancient wrongs. Damn the English and their dictionaries.” The more you read, the more you descend into the rhetoric and evidence of conspiracy theorists. Baseball writers used “jazz” early, baseball players were largely Irish, they went to Hot Springs Arkansas where there was an Irish population, they used the hot springs and came back to California talking about “jazz” as heat and energy – what more do you need for proof you English bastards? And don’t get me started about St. Patrick and the snakes, which is just a metaphor for the Catholic oppression of pagan snake goddesses…
Okay, fine. I think it is possible and even likely that some of these folk etymologies from Ireland are the accurate explanations, though we may never get hold of good supporting data. I’ll even grant that many nonsense syllables in folk songs are merely poor renditions of Irish Gaelic phrases, such as Billy mee-oo ree-I- ree ay from “Pat works on the Railway” really means “I get up in the morning;” or whoopee-ti-yi-yo comes from something about underfed calves. I don’t claim that they are, but I allow that it’s possible.
I just don’t get the certainty, and the language of victimhood about the whole thing. The card game faro might come from Gaelic “the turn.” Yet who plays Faro anymore? Describing rich or respectable people as “swells,” which is familiar to readers of 19th C literature, could indeed be Irish. Who cares? Even readers of 19th C literature don’t care. The Dead Rabbits, a NYC gang made famous in the Scorcese film might actually mean “very big hulking fellows.” Sure and that’s goin’ t’ change the world naow, isn’t it Michael?
Word origins are for fun. What’s the Gaelic for “Get a life?”
*Cf. Mickle, an obsolete word for "many," or the place name in Tolkien Michel Delving, meaning "many diggings."