A hangover that is so bad you can't even remember your phone number, pin number, etc.
I drank so much last night, I can't even remember where I parked. I've got some serious Brain Cloud today.
When you are getting a blowjob
from a girl and instead of cumming in her mouth, you pull out and shoot a load in her nose. It goes into her brain and explodes and oozes around like a "brain cloud".
Dude, I was totally getting a hummer from that cheerleader you did last week and instead of cumming in her mouth like you did, I totally pulled out and shot her a brain cloud up the nose!