A while ago I came across a very interesting article by Brendan Patrick Keane on Irishcentral.com which reviewed a book by the late Daniel Cassidy called “How the Irish invented Slang”. The link to the article is below. I have yet to get a copy of this book, and I am really looking forward to reading it as what was covered in the article was very intriguing.
The message of the book is that the whole English-speaking world has been using words derived from Gaelic without knowing it. While there are certain Irish-derived words that I already knew about (e.g., smithereens and galore), this article unveiled the Irish origin of so many other well-known slang words that Hollywood has been teaching us for decades.
The explanation given for these words becoming part of American parlance is that Irish emigrants who arrived in the US after the famine were predominantly from Gaelic-speaking areas. As poor emigrants often graft to get ahead, having a secret code language derived from Gaelic would be useful for hiding illegal activities from the authorities.
Here is a list of some of the slang words in the article:
Ballyhoo: from “Bailliú”, meaning “Gathering”
Big Shot: from “Seoid” meaning “Jewel”
Buddy: from “Bhodaigh”, meaning “Pal”
Cop: from “Ceapaim”, meaning “I catch” or “I think”
Darn: from “Daithairne ort”, meaning “Misfortune on you”
Ditch: from “De áit” meaning “Leave” or “De-place”
Gee whilikers: from “Dia thoilleachas”, meaning “Will of God”
Gee whiz: from “Dia uas”, meaning “Noble God”
Holy cow: from “Holy cathú”, meaning “Holy grief”
Jiffy: from “Deifir”, meaning “Hurry”
Joint: from “Díonteach” meaning “Penthouse”.
Mug (as in “Ugly mug”): from “Muic”, meaning pig
Racket: from “Reacaireacht”, meaning "Dealing, selling or gossiping."
Slogan: from “Slua Ghairm”, meaning “Battle cry”
Slum: from “'S slom é, meaning "It's bleak”
Snoot: from “Snua aird”, meaning “Nose in the air”
Swank: from “Somhaoineach”, meaning “Valuable”
Swell: from “Sóúil”, meaning “Luxurious”
You dig?: from “An duigeann tú?”, meaning “Do you understand?”
https://www.irishcentral.com/roots/history/irish-words-litter-new-york-slang