A fork of Rural Dictionary
Noun: When the water trap beneath a shower or bath becomes clogged up with a build up of matted rotting hair, skin cells, palm oil, and various bodily secretions. That's a Johnson reservoir.
Johnson reservoir: Oh man, my shower was draining really slowly today, so I had to partially dismantle the piping to clear the blockage. Turns out I had a Johnson reservouir under there!!