Green’s Dictionary of Slang

a mixture composed of equal parts of stout and porter.

1821		‘A Real Paddy’ Real Life in Ireland 43: A breechless spalpeen, who flew to Jones’s and returned with a cooper of crusty.
c.1851		‘New Beer House Act’ in C. Hindley Curiosities of Street Lit. (1871) 92: Any keeper of any refreshment house who shall have the cheek to sell [...] one glass of cooper.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1869		Man about Town 16 Oct. 44/1: We have often heard ‘drinks’ asked for at restaurants by funny names. Thus ‘Stout and Porter’ is always called for as ‘Cooper,’ and ‘Old and Bitter’ as ‘Mother-in-Law’.
1877		J. Diprose London Life 42: His favourite beverage is a ‘pot o’ four arf,’ or ‘drop o’ cooper’.
1909		J. Ware Passing Eng. of the Victorian Era 91/1: Cooper (Peoples’). The name of a beer-mixture of common beer (3d. per quart) and stout (6d. to 8d. per quart).