Puddles of disease-ridden and stinky body fluids left behind by homeless people.
The puddles could be anywhere, but are most often encountered on public transit systems.
The "juice" is an unknown mixture of any of the following: urine, diarrhea, rancid sweat, dirt, crabs, lice, blood from open sores.
Dude: I barely made my BART train and there was only one seat left. Always check the seat before you sit down. I sat on the damn spongy seat in a big warm puddle of hobo juice.
Dude 2: Damn dude, that's hella nasty. I'd check with the doctor to make sure you're not infected.
Dude: Yeah, I had to go home and shower. I wasn't going to go to the party smelling like hobo juice cologne.