Green’s Dictionary of Slang

country cousin n.¹

a dozen.

1909		J. Ware Passing Eng. of the Victorian Era 95/1: Country Cousin (Rhyming). A dozen, e.g., ‘They put away about three country cousins o’ Bass.’.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxiii 4/4: country cousin: Dozen. Usually shortened to country.
1971		J. Jones Rhy. Cockney Sl.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 7: The Duke of Kent was needed and she needed to pull at least a half a country cousin of little brown jugs. [Ibid.] 33: Kissing Cousin Dozen.
1999		G. Seal Lingo 91: A country cousin is a dozen, usually bottles or cans of beer.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.