Tonic water (or Indian tonic water) is a carbonated soft drink in which quinine is dissolved.
Quinine was added to the drink as a prophylactic against malaria, since it was originally intended for consumption in tropical areas of South Asia and Africa, where the disease is endemic. Quinine powder was so bitter that British officials stationed in early 19th century India and other tropical posts began mixing the powder with soda and sugar, and a basic tonic water was created.
The first commercial tonic water was produced in 1858. The mixed drink gin and tonic also originated in British colonial India, when the British population would mix their medicinal quinine tonic with gin.
So since we’re here, it would be rude not to. Chin, chin!