Acually, most swords do weigh on average, for a medieval sword, 15-50 pounds, depending on make and type.
Medieval reenactment is my hobby. I have held and used swords of several different types ranging from Italian rapiers to a Scottish claymore and a German zweihänder. A sword weighing more than five pounds is unusual; to even
think that someone could lift and use a fifty pound chunk of steel as a
club, much less a sword, for any length of time is ludicrous.
Just to get a sense of how heavy fifty pounds is, and how large a fifty pound sword would be:
Steel has a density of 0.284 lb/in³; that is, a pound of mild steel has a volume of roughly three and a half cubic inches (0.284x=1; solving for X gives 3.52116, continuing on the way irrational numbers do). For simplicity's sake, let's say the cross section of the sword in question measured perpendicular to the blade is a square inch. (For any sword I'm aware of it would actually be less than that by a significant margin, but for simplicity's sake we'll say that.) In this admittedly simplified case, to attain a weight of fifty pounds the sword would have to be
176 inches long, or roughly 14.6 feet. Keep in mind, that's assuming an average cross section area of a full square inch; this would be a gigantic blade even discounting the fact that most swords taper. Even a very large blade like that of a zweihänder or other great sword would not have a blade that wide and thick. (Recall that a square inch can be one inch on all sides, or two inches on two sides and a half inch on the other, and so on; most swords are no thicker on their thinner axis than an average of a quarter inch to be generous, so we're assuming a four inch wide blade for the calculations, something that does not exist as a practical blade.) In actual fact, such a blade with a typical cross section of less than half of that would of course be more than twice as long. I don't fancy trying to wield a thirty foot sword; do you?
And now for just how heavy fifty pounds is. I'm a swordsman for fun, but I'm a baker by trade. Every day I handle fifty pound bags of flour at work. They are
heavy, and I'm no weakling. I can pick them up and carry them around for quite some time, to be sure, but holding something over your shoulder and swinging it around at the end of your arm are two entirely different physical feats. And that's before we consider that the fifty pound bag is compact; with the sword you have most of that weight on one end of a thirty foot lever, the fulcrum point for which is your hand (forward hand if you're using at two handed grip). A basic familiarity with physics should tell you all you need to know about what happens with that. Hint: It's the same principle that drives another medieval weapon, the catapult.
Also,
this.