Jackeen is a mildly pejorative

Pejorative

Pejoratives , including name slurs, are words or grammatical forms that connote negativity and express contempt or distaste. A term can be regarded as pejorative in some social groups but not in others, e.g., hacker is a term used for computer criminals as well as quick and clever computer experts...

 term for someone from Dublin, Ireland

Republic of Ireland

Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...

. 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

John (given name)

John is a masculine given name in the English language. The name is derived from the Latin Ioannes, Iohannes, which is in turn a form of the Greek , Iōánnēs. This Greek name is a form of the Hebrew name , , which means "God is generous"...

 (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

Dublin GAA

Dublin County Board of the Gaelic Athletic Association , or Dublin GAA, is one of the 32 county boards of the GAA in Ireland, and is responsible for Gaelic games in County Dublin. The county board is also responsible for the Dublin inter-county teams...

players and supporters. The term has also been shortened to "Jack" or the plural "Jacks" by Dublin fans as a way to refer to themselves.