A little bit of history of the Pimm's Cup before we get to the recipe:
James Pimm, a farmer's son from Kent, became the owner of an oyster bar in London. One of his signature drinks was a tonic which included a gin-based drink containing a secret mixture of herbs and liqueurs to aid in digestion. He served it in a small tankard that was then known as a No. 1 Cup. That led to the name of the liqueur known as Pimm's No. 1.
It's one of New Orleans' Napoleon House's signature drinks.
Fill a tall (12 oz.) glass with ice and add 1 1/4 oz. of Pimm's No. 1
Add 3 oz. of lemonade
Top with Sprite, Ginger Ale or Ginger Beer
Garnish with a cucumber slice