What does go suck a lemon mean?

Answer 1

In the Age of Sail, scurvy was a major problem for sailors on long sea voyages. Early 17th century physician John Woodall advocated the practice of sucking on Oranges or lemons as a preventative measure to avoid the vitamin C (ascorbic acid) deficiencies that cause scurvy.

Fast-forward almost ninety years to the War of the Third Coalition and specifically the Battle of Trafalgar, the decisive sea battle between England and Napoleonic France. At the start of the battle, Admiral Horatio Nelson (the one-eyed dude whose statue looks over Trafalgar Square in London) asked for Admiral Villeneuve's surrender. The French commander reputedly told Nelson to "go suck a lemon", perhaps as a way of suggesting Nelson was addled by disease if he thought the French would give up so easily. Anyway, Nelson died from a musket ball during the battle, but the French lost the battle and with it any hope of overcoming English sea power and invading the British Isles.

Answer 2

Basically, Go away and do something unpleasant to yourself.