The real meaning behind "best man" is not what you might think. Back when the bride did not have much say in whom she married, the “groom would bring his strongest and most trusted friend” - just in case she decided to run off before getting married.
The best man was also there in case the family of the bride tried to stop the wedding because they were not happy with whom she decided to marry.
The word ‘Best’ refers to his skill with a sword.
The best man is there to stand guard for the groom and to help protect him, just in case it is needed. Today, though, the best man is there to support his friend or brother in marrying the love of his life.
Via Dream Wedding