Lemon sour is a type of soda made by various vendors, often used in many mixed drinks. There are also alcoholic variants sold in Japan.
The Schweppes brand of lemon sour[1] is sold by Coca-Cola. Canada Dry's lemon sour was sold by Dr Pepper/Seven Up.
Lemon sour is also a popular soda in Japan, where is it sold as an alcopop by Suntory (Kodawari Sakaba no Lemon Sour) and Coca Cola (Lemon-dou).[2] [3]
Lemon sour was referenced in the 88rising album "Head in the Clouds", specifically in the song Japan 88, featuring Famous Dex, Keith Ape and the one who mentioned it, Verbal.