ha ha! From Sister Act to Cistern Act.
It is my understanding that a cistern is a place to store rainwater for later use. I respectfully suggest that a cistern is not where Whoopi has gone. A septic tank, on the other hand.............
I think you're right, but see #2 below. I concede that 'septic' is probably a better choice for Whoopi.
cistern
n 1: a sac or cavity containing fluid especially lymph or cerebrospinal fluid [syn: cisterna] 2: holds the water used to flush a toilet [syn: water tank] 3: an artificial reservoir for storing liquids; especially an underground tank for storing rainwater