Contrary to popular belief, this phrase, taken from Spanish, does not mean one on one or man to man. It means, literally, hand to hand. The implication is that one should put down their guns, knives, swords or the like and put up their dukes, the idea being that this will smoke out which of the two combatants is the "real man". It's also rather less deadly than most forms of armed combat, but this is usually not the point.