NFFNSNC is a Latin abbreviation, frequently found inscribed on Roman grave markers, which stands for non fui, fui, non sum, non curo ("I was not, I was, I am not, I don't care.").
The phrase is associated with the ancient Greek philosopher Epicurus, and is often found on the gravestones of his many Roman followers. Epicurus famously held that death was meaningless and not to be feared.
The fact that this phrase was reduced to an abbreviated series of letters - perhaps implying that it was perceived as a mere cliché - has been used, especially by latterday Christian writers, as evidence that cynicism and despair were endemic to Roman culture, and used it to compare Roman pagan beliefs disfavorably to the Christian belief in a heavenly afterlife. One Baptist preacher, a certain Harry Emerson Fosdick, even declared the phrase "the essence of surrender."
Although this type of attack has been numerous and enduring, I find it fairly ludicrous, especially given the prevalence of the acronym "RIP" on Christian grave stones. Stone is expensive, and carving it takes time, so abbreviations of all sorts are common on all manner of gravestones. Moreover, the phrase to me signifies more of a laughing in the face of death and thus snatching a small measure of triumph away from death, rather than a meek surrender to death.