Jackeen is a 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.
The term Jackeen is believed to be derived from the name Jack, a common English nickname for the names James and John, or in reference to the Union Jack, the flag of the United Kingdom. Following the Norman invasion of Ireland beginning in 1169, Dublin became the centre of the Pale, the part of Ireland directly under the control of the English government in the late Middle Ages. By the 19th century, Dublin had served as the centre for English rule in Ireland for centuries, and Dubliners were stereotyped as being heavily Anglicized and considered the most English of all the Irish.
Jack is combined with the Irish diminutive suffix "-een" (represented as -ín in Irish) meaning little, commonly found in Irish female names such as Roisín ("little Rose") and Maureen (Mairín, "little Mary"), implying that Dubliners are "little Englishmen".