A sarcophagus is a stone coffin. The plural form in Latin is sarcophagi, and in English it is sarcophaguses. Both are correct, though sarcophagi is by far the more accepted spelling.
The original Greek word sarkophagos meant “eating flesh”, and was usually paired with the word lithos or stone. They thought a certain kind of limestone of the coffin was responsible for dissolving the corpse inside. Soon sarkophagos by itself was used for any kind of coffin.