I've only ever armwrestled with friends or people at school =p Usually, I win, but that's because I haven't faced anyone who is actually awesome at it yet.
Depending on your technique during armwrestling, you will screw your arm over so you gotta be careful. There's more techniques to it than people even realize.
Btw -- in case you are interested, there's actually a movie about arm wrestling called Over the Top:
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I've never seen it before, but I do know the guy constructs some sort of weird arm wrestling training machine that he has in his truck haha. And since it has Sylvester Stallone in it, you get to make fun of him the whole time!