To urinate with a partial errection.
(Typically happens in the morninig in conjuction with Morning glory)
Chubby = the partial errection
Rain = the urine spraying everywhere
"oh dude i got up this morining and had morning glory, i went for a piss and i had damm Chubby rain, urine everywhere bro."
a crime scene investigator term used to describe the phenomenon were in an obese or overly fat person dies in a room and is not found for quite some time. the result being that the large fat deposits and bodily fluids start to evaporate and condense on the ceiling, so when someone finally does find the body the first step into the room results in jaring the dense semi-liquid substance from the ceiling downward onto the unfortunate bystander.
CSI#1/ dude what happened to you in there?
CSI#2/ the second i walked into the room fucking chubby rain man.