Contrary to popular belief, 7-Eleven did not invent its well-known slushy frozen drink. The slurpee was a happy accident of Omar Knedlik, a Dairy Queen owner in Kansas City during the 1950s. When Knedlik's soda fountain broke down, he improvised by putting some soda bottles in the freezer to stay cool. He served the not-fully-frozen bottles to customers who ended up loving the consistency of the cool treat.
Knedlik then created his own machine that added carbon dioxide to make the drink fizz then held a naming contest. The drink became known as ICEE. In 1965, 7-Eleven licensed Knedlik's machine and their ad agency director named the drink Slurpee® after the sound made while drinking it through a straw. To date, 6.5 billion Slurpee® beverages have been sold through the store alone and approximately 13 million people sip Slurpees each month, according to 7-Eleven.
Slurp on that!
Info from Grandparents.com