A somewhat literary or formal term for a dead body. In everyday speech, 死体 (したい) is more common.
Many corpses lay on the battlefield.
The novel begins with a description of heaps of corpses.
heaps of corpses all around
Compound of 死 (し, 'death') and 屍 (し, 'corpse'). The reading しし is a straightforward combination of the on'yomi of both characters.