Literally to consume ones own excretions - a byproduct of their biological
existence - in a manner akin and even more vile than
cannabalism - and then promptly cease to exist; it is not simply because they have eaten the shit that they die but because in the grand scheme of things they are eternally damned and will simply perish by the will of the universe, presumably.
Although the Russian insults to call someone the pus from the zit on the ass of a
diseased camel is quite poetic and rich, the English phrase "eat shit and die" is in the Anglo tradition a more perfunctory and
utilitarian insult which is at the same time
existentially profound and shockingly curt.