Green’s Dictionary of Slang

keeping cully n.

[SE keep + cully n.1 (2)]
[late 17C–early 19C]

1. ‘one that maintains a Mistress and parts with his money very generously to her’ (B.E.).

2. (also keeping-cit) ‘one who keeps a mistress, as he supposes, for his own use, but really for that of the public’ (Grose, 1785); thus keeping lady, a mistress/prostitute [cit n. (1)].