Green’s Dictionary of Slang

merry-go-down n.

(strong) beer.

1618		The Owles alamanacke 43: The best medicine for the fleas will be a cup of merry-goe-downe, and the onely helpe to clap the doore vpon sorrow, and shut him out, will be a draught of March beere.
1669		‘In the Praise or Ex-Ale-tation of ale’ in An antidote against melancholy 1: I Mean not to tast, though thereby much grac’d, [?]or the Merry-go-down without pull or hale.