The Left hand shake goes way back to the origins of Scouting, and was inspired like many original Scouting concepts by B.-P.'s Army career.
When Captain Baden-Powell entered the capital city of the Ashanti people in 1896 he was met by one of the chiefs who came to him holding out his left hand. B.P. held out his right hand in return but the Chief said "No, in my country the bravest of the brave shake with the left hand.
This was because African Warriors typically carried a spear in the right hand and a shield in the left. To shake left handed meant you had to put down your shield and put your life in the hands of the other person.
So began the Left hand shake of the World Wide Brotherhood of Scouting.
-- Thanks to Jim McGregor, 1st Camden South Scout Group, Camden NSW Australia