What is Thunder Cup?
A plumbing fixture devised for the disposal of bodily wastes, including urine, feces, methane and vomit.
It usually comes in the form of a porcelain receptacle, adorned with a wooden or plastic seat which one sits on / squats over / aims into / puts head down, depending on the activity.
Should never, ever, be used to wash clothing in, or as a source of fresh drinking water.
See also: defecation station, great white telephone, groan throne
"Man, that curry last night was HARSH - I just destroyed the thunder cup!"