Growing up Irish in Queens and on Long Island, Daniel Cassidy was nicknamed Glom.

“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.