Jackeen is an Irish insult with an English origin. The noun Jack has been used to refer generally to a man, and especially an ill-mannered or obnoxious fellow, since at least the 1600s. (Shakespeare uses this sense of the word in The Taming of the Shrew: “A mad-cap ruffian and a swearing Jacke.”) The Irish paired the English jack with the Irish diminutive suffix -een (from the Gaelic -īn), which literally translates to “little jack,” just to add insult to injury. Jackeen is used to refer to, as our Unabridged Dictionary puts it, “an obnoxious self-assertive dude.”