Green’s Dictionary of Slang

(UK Und.) strong beer.

1677		R. Hawtrey Letters (Egerton Ms.) 2716: We will drink your healths both in stoutt and best wine [OED].
c.1698		B.E. Dict. Canting Crew.
1719		in D’Urfey Pills to Purge Melancholy V 83: We will frolick in Stout, And banish all Care in a Mug.
1720		Swift Poems, To Stella 32: Or kindly, when his credit’s out, Surprise him with a pint of stout .
1755		Johnson Dict. Eng. Lang. (1785) n.p.: stout. A cant name for strong beer.