An unmarked grave is one that lacks a marker, headstone, or nameplate indicating that a body is buried there. However, in cultures that mark burial sites, the phrase unmarked grave has taken on a metaphorical meaning.
As a figure of speech, a common meaning of the term "unmarked grave" is consignment to an ignominious end. A grave monument (or headstone) is a sign of respect or fondness, erected with the intention of commemorating and remembering a person.
Conversely, a deliberately unmarked grave may signify disdain and contempt. The underlying intention of some unmarked graves may be to suggest that the person buried is not worthy of commemoration, and should therefore be completely ignored and forgotten, e.g., Seung-Hui Cho, Dylan Klebold, Adam Lanza.
Unmarked graves have long been used to bury executed criminals as an added degree of disgrace. Similarly, many 18th and 19th century prisons and mental asylums historically used numbered (but otherwise featureless) markers in their inmate cemeteries, which allowed for record-keeping and visitations while also minimizing the shame associated with having one's family name on permanent display in such a disreputable context. Plot E at Oise-Aisne American Cemetery (consisting entirely of soldiers executed for rape and/or murder) is a rare example of this policy persisting into the 20th century. More recently, the practice has been to cremate and secretly scatter the ashes of notorious criminals in some anonymous place. This was the fate of Nazi war criminals such as Adolf Eichmann, Hermann Göring, Heinrich Himmler, Fritz Sauckel, and Julius Streicher. The remains of British serial killers Myra Hindley, Dr Harold Shipman, and Fred West were treated in the same way. Cremation and secret scattering of the ashes has the additional effect of removing all possibility of there being a grave to visit in the future.