Green’s Dictionary of Slang

an Irish immigrant.

1740		Ordinary of Newgate His Account 6 Aug. 15: I quitted their Company, and got acquainted with one we call the Grecian.
1824		J. Wight Mornings in Bow St. 116: [Y]e Irish Grecian, says he — come out o’ that and I’ll give it to ye!
1833		‘Life In London’ in Swell!!! or, Slap-Up Chaunter 11: The yokels all are floor’d with fright, / So full of yawns and gazes; That up to town to see the sight, / They’re pouring in like blazes: / To put these Grecians on their pins, / That they may come to time, sir.
1977		Maledicta 1:2 134: An obsolete example of the lucus-a-non-lucendo use of words is grecian for an Irish immigrant, and hence for any uncouth newcomer; perhaps Irish migrants were sarcastically called grecians because they lacked any of those cultured qualities that set the true Athenian apart from the Boeotians [i.e. stupid peasants].