Green’s Dictionary of Slang

(US und.) the lowest grade of tavern or drinking venue.

1853		Life in Boston & N.Y. (Boston, MA) 1 Oct. n.p.: You are progressing fast. From keeping an r.g. crib, you have converted your shop into a place of assignation.
1855		Life in Boston & N.Y. (Boston, MA) 14 Apr. n.p.: Tom got rather ‘balmy’ the other day [...] He had better keep away from that low r.g. hole.