Definition: A Soylent Grin is the canned grimace of a soyboy who learns soylent green is people, and doesn’t even bother to feign horror before returning to his consumption.
He knows his fate is to be eaten in turn by the next generation of soyboys, each generation more neotenous and precocious in its development trajectory than the last (ending in the theoretical “Soy Singularity” where infants feed on each other in the womb). In the extreme case, he even takes a perverse pleasure in his consumption because he feels that in cannibalizing the next generation’s future this “time paradox” will break the chain of causality which dooms his subspecies. This understanding of causality can probably be explained by R-selection, which produces an IQ approaching that of an unmixed homo sapiens sapiens in sub-Saharan Africa; to wit, about 65. If you look too long into his gaping maw, you will see the abyss of his soul looking back.