According to its website, here is the history of the famous drink:
London's dandiest city gents loved to partake in oysters and gin. But with its bitter tang, gin was knocked back, not savoured. Enter shellfish-monger James Pimm and his famous central London 'Oyster Bar' in 1823. Patrons soon swallowed oysters with the PIMM'S 'house cup'. Flavoured with liqueurs and fruit extract, this more palatable long drink 'gin-sling' kick-started the great British PIMM'S story.
According to Wikipedia, there were several variations over the years:
Historically, there have been 6 types of Pimm's, each given a number corresponding to a different alcoholic base (gin, scotch whiskey, brandy, rum, rye whiskey and vodka respectively). Only Pimm's Number 1, 3 and 6 are still available.