Green’s Dictionary of Slang

my dirty cousin phr.

a pej. form of address, usu. aimed at one who is judged to be affecting modesty.

[UK]Swift Polite Conversation 67: A Kiss! marry come up my dirty Couzin: Are you no sicker?
[Ire]Cork Examiner 13 Apr. 4/2: He pretends to be disgusted with the Prince of Wales ‘for taking the liberty of calling me Charles!’ Marry come up, my dirty cousin!
[Scot]Fife Herald 5 Apr. 4/5: Who will readily forget the strange scorn [...] scattered through that letter where he speaks of the prince with a ‘Marry come up, my dirty cousin’?
[UK]Cornishman 15 Sept. 2/3: Mr Tredweeke: Get on, my dirty cousin.