The "weight" of anything is
(the object's mass)
times
(the acceleration of gravity in the place where the object is).
Let's back up for a second, and consider why the hammer is called
"a 2-pound hammer" in the first place ?
That's because on Earth, where the hammer has spent all of
its life so far, and where it is acted upon by Earth's gravity, it
weighs 2 pounds.
The acceleration of gravity on Earth is 9.8 m/s².
(weight) = (mass) x (acceleration of gravity)
so
2 pounds = (mass) x (9.8 m/s²) .
Look at that equation.
If the left side suddenly becomes 1 pound instead of 2 pounds,
then something on the right side must also have suddenly become
1/2 of what it used to be. It's not the mass ... mass doesn't change.
So it must have been the acceleration that changed by half ...
it became (1/2 of 9.8) = 4.9 m/s² .
