The use of "slimline" to describe sugar-free tonic water in the UK, rather than "diet," may stem from historical branding decisions. Schweppes introduced its slimline tonic in 1965, identifying a market for a low-calorie mixer.
This launch occurred shortly after Patio Diet Cola was test-marketed in the US in 1963 and rebranded as Diet Pepsi in 1964. Coca-Cola did not adopt Diet Coke until 1982.
Schweppes had been producing carbonated mineral waters since 1783 and by the 1940s, marketed its tonic water as a mixer. The company's slogan "Schhh … you know who" aimed to boost brand recognition and appeal to female drinkers.