1. In a wedding party, the next ranking groomsman after the best man. Often the X-factor at the bachelor party, and wedding reception responsible for turning things up a notch.
2. On a gang or team, the third ranking member. Often a wild card or tough guy.
"After winning and then losing 3 grand at the blackjack tables, I brought a couple strippers back to the bachelor party suite, and finished them both off. Just another night glorious night as a left hand man."
Historical examples: Mr. T in The A-Team, Zach Galifianakis in The Hangover, Dennis Rodman on the Chicago Bulls, Raphael from 'Teenage Mutant Ninja Turtles', Will Ferrel in 'Old School', Booger in 'Revenge of the Nerds',