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Jackeen is a mildly pejorative term for someone from Dublin, Ireland. The Oxford English Dictionary defines it as "A contemptuous designation for a self-assertive worthless fellow," citing the earliest documented use from the year 1840.
Derivation and meaning
The term is derived from a nickname for John (i.e. Jack) from the Union Jack (As Dubliners were considered the most English of all the Irish) combined with the Irish diminutive suffix "-een" (meaning little)  (-ín in Irish) found in many Irish female names such as Roisín ("little Rose") and Maureen (Mairín, "little Mary"). Today, Jackeen is often used to describe Dublin GAA players and supporters.