in Roman religion, "spirits of the dead considered as tutelary divinities of their families," from Latin manes "departed spirit, ghost, shade of the dead, deified spirits of the underworld," usually said to be related to Latin manus "good," thus properly "the good gods," a euphemistic word. De Vaan cites cognates Old Irish maith, Welsh mad, Breton mat "good." The ultimate etymology is uncertain (compare mature).
Three times a year a pit called the mundus was officially opened in the comitium of the Roman Forum, to permit the manes to come forth. The manes were also honored at certain festivals, as the Parentalia and Feralia; oblations were made to them, and the flame maintained on the altar of the household was a homage to them. [In this sense often written with a capital.] [Century Dictionary]