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.
References
Jackeen Wikipedia(Text) CC BY-SA