About a pound per hand in American, around 400 g for the American scientists and drug dealers (and everybody else in the world)

Generally you get a good estimate by assuming a body part is just water, and then look at its volume. My fist here is about 400 mL and the other comment said the average is 400 grams. Now I'll look at my whole arm... 3L? I found a source that says one is about 5% your body weight, so 70 kg • 0.05 = 3.5 kg. Pretty good!