Green’s Dictionary of Slang

a party at which there is no form of intoxicating liquor.

1883		G.A. Sala in Illus. London News 10 Mar. 235/3: One might have imagined the symposium to be more of the nature of a ‘lemon squash’ party or a ‘Botany beer’ one. Botany Beer, it has recently been decided on judicial authority, is not beer at all.
1909		J. Ware Passing Eng. of the Victorian Era 44/1: Botany Beer Party (Soc., 1882). A meeting where no intoxicants are drunk.