Hang brain is not the act of placing your scrotum on someone or something as noted in the other definitions. That would be "tea baggin." Hang brain is just the act of hanging your scrotum (testicles and all) out of the zipper of your pants.
The doctor wanted to see check my scrotum for unusual lacerations, so he asked me to hang brain.
Events, usually sponsored by NAAFA for obese men and women (Mostly women) to get together and wear their most outrageous club attire without the fear of being ridiculed by normal sized, more attractive people. A gold mine for chubby chasers.