unvisited grave; grave with no mourners
Refers to a grave that has no family or relatives to care for it or mourn the deceased, often due to the breakdown of family ties or the deceased being unknown.
It is said that the increase in unvisited graves is because family ties are weakening.
Compound of 無縁 (muen, 'without connection/relation') and 墓 (haka, 'grave'). The reading ばか is a rendaku form of はか.