Get a stick and tie some rope to it in the middle. Get you a couple of old, rusty 10 pound plates. Tie rope through center of plates with about 3' of space from stick to plates.
Grab stick with both hands (palms down) having each hand equal space from knot tied to stick. Pick up slightly so plates aren't touching floor. Proceed to work hands in twisting motion getting rope around stick until plates are up to stick. Repeat until your forearms are toast.
There is actually a civilized one of these you can find in most sporting goods stores.
I do a lot of forearms like this for grip strength. One of the best ways to increase punching power and hand strength in BJJ.