to use every possible insult you can when verbally lashing someone until they are completely and utterly served and owned. "Unload" refers to someone "unloading" a clip on someone with a gun, releasing all possible ammo into the other person, resulting death (probably.) This is pretty much the same, except verbally.
Person 1: "My girlfriend thought it would be cool to ask another guy to Prom, so I unloaded on her in front of everyone at lunch. I feel much better."
Person 2: "Wow...dude....you rocked her....I didn't know you had that much hate inside you...."