Just curl the weight with your fingers just using the barbell with a moderately heavy weight-you'll feel your forearms burn like hell. In arm wrestling you can use this exercise to strengthen your finger strength so you can grab or "cup" your opponent's hand.

Hammer Curl Grip Preacher Lowers

Exercises like this are very specific to arm wrestling. Pining down your opponent's hand mimics this arm position. Due to the fact that this is a static exercise, you can use your whole body on it. Strengthen your whole arm and wrist by keeping them straight. As you work your biceps, shoulders, and forearms, you will be able to build your arm strength. You could end up in a fitness fails compilation video on YouTube or Instagram, as it is an extremely strange exercise.

Barbell Wrist Curls

Place a barbell with a somewhat light weight on a bench. Lie on your back with your elbows between your legs or on your knees. You need to be comfortable in each position and with the barbell. Curl the barbell in the direction of your wrist. One of the harder variations is the reverse wrist curl, where your fist is raised towards your wrist as you grip the barbell with an overhand grip. Take into account both wrist strength and mobility, as well as forearm size.

Pronation Forearm Flex

This is the only exercise I could come up with that sounded cool, so you're stuck with it. This can be done with a dumbbell or a belt and a plate. Ideally, if you want to do this with a dumbbell, you should choose one that can be loaded on one side and left empty on the other. Take an easier dumbbell instead. Hold the dumbbell with one hand, then turn it over to the other side while resting your arm on the bench. You should rinse each side ten times. Wrapping a belt around a heavy plate then turning your wrists from side to side is all it takes. This technique is used by armwrestling legend Devon Larratt.

Sand Grabbing

A variety of strength athletes around the world use this one, including Shaolin Monks (yes, I'm serious). Grab a bucket of sand and fill it up with as much as you can. You won't be able to grab any sand the deeper you dig and even if you do, the sand will fall out of your hand. Forearm size and grip strength are developed this way.

Quick Wins to Build Forearm Strength

Invest in a set of grips, like Gripzilla Ultimatum Gripzilla Ultimatum was created with the goal to your grip and overall body strength substantially. This kit was made as a better alternative to the entry-level plastic/rubber grippers that we're all familiar with but need a challenge or a level up and enhanced durability. 

Improves endurance and reduces fatigue in overworked hands Helps speed up recovery after arm injuries or strokes Boosts blood flow and joint health.