Its when you mix Sprite & Dr. Pepper. It's not always a good thing usually gross. It describes someone who is not a "Heather"/Plastic (from mean girls)
coined by the Drag Queen 'Carmen Carrera' who was a contestant on RuPaul's Drag Race Season 3 EP 12
n. A "Sprepper" is a mixture of Sprite and Dr. Pepper. It's really sweet but that's not necessarily a good thing and usually leaves a bad taste in your mouth. When used to describe a person, it's an insult.