Green’s Dictionary of Slang

rum or gin.

1789		G. Parker Life’s Painter 129: I say, call for a bobstick worth of rum slim [Ibid.] 162: Bobstick of rum slim. That is, a shilling’s worth of punch. Slim is cant for punch.
1827		Commercial Advertiser (NY) 1 Feb. 2/3: After roystering at the Theatre, they broomed to a neighboring bousing ken, [...] and here, having drunk couge and slim till they were hocus.
1833		‘The Christening of Little Joey’ in Corinthian in Spedding & Watt (eds) Bawdy Songbooks (2011) IV 46: I am just going to drink some slim.