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...."
Manny: Hey I just whacked this fucker out for about 15 grand in diamonds, do you know somewhere I can unload these?
Larry: Yeah, call up Lenny the Jeweler. He'll give you the best price for it.
Using stored up but unused talents and abilities to the maximum in oneepicaction.
Kevin had been waiting for three quarters for his dumb basketball coach to finally put him in the game so he could show what he could do. When the starting point guard got injured, the coach put Kevin in the game, where he proceeded to "unload your nuts" all over the other team by scoring 75 points in one quarter.