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.