Commonly the groom's brother or best friend, the best man stands beaming next to the husband-to-be. Sometimes he even cries. But yesteryear, he would've held a sword. You know why he stands so close to the groom? So he can intercede with his blade if needed. So where does that "best" part come in? He was literally the groom's best swordsman.
This tradition hails back to the day when a wedding was a financial transaction -- and as we all know, sometimes financial transactions can go bad. Should the bride's father have second thoughts or a lovelorn rival spring from the rafters, it was the best man's job to ensure the deal went down as planned.
If kidnapping became necessary, the best man was the muscle. Later, he stood guard outside the nuptial bedroom.