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.