Also, lats, for back pressure. The greatest arm wrestler ever John Brzenk said pullups was the best exercise you could do for improving arm wrestling, besides actual arm wrestling. He was really adamant on how much you have to arm wrestle to become great at it. Nothing beats actually practicing day after day.
Brzenk was a pretty jacked dude (even by weightlifter standards) but he beat some absolute mountainous men with his superior technique. He has an awesome quote too which is something along the lines of "My arm is like electricity, it just finds the path of least resistance".
Also, the documentary (Pulling John)is pretty good. I have watched it twice, it is on youtube and Netflix. Highly recommend it even for people who are not into arm wrestling. Just a very cool look at the niche sport and all the interesting personalities in it over the years.