Properly the name of the halls of the Vala Námo, the dwelling place of departed Elvish spirits (S/28). It is often used as the name of the Vala himself, however. It is a compound of mando “custody” and the suffixal form -os for osto “stronghold”, so that its stem form is Mandost- (MR/350). In one place Tolkien translated the name as “Castle of Custody”, but said this was only an approximate meaning (MR/350).
Conceptual Development: This name dates back to the earliest Lost Tales. Even at this early stage, ᴱQ. Mandos was used to refer to the Vala but was said to be properly the name of his halls, though at this stage his true name was said to be ᴱQ. Vê instead of Námo (LT1/66, QL/58). At this early stage the name was glossed “Hell” and given as a derivative of the root ᴹ√M(B)ṆÐṆ “bind” (QL/58), and appeared sometimes in the variant form Mandor (PME/58, 99; PE15/73).
The name ᴹQ. Mandos appeared in The Etymologies from the 1930s with the gloss “Dread Imprisoner”, derived from a combination of the roots ᴹ√MBAD “prison” and ᴹ√GOS “dread” (Ety/GOS, MBAD).
His true name Námo did not emerge until Silmarillion revisions from the 1950s-60s (MR/150). Elsewhere in the Silmarillion revisions from the 1950s-60s, Tolkien briefly considered changing this name to Mandar, but this change was rejected (MR/205).
Mandos (Mandost-) noun "Castle of Custody" (the approximate meaning, according to MR:350). Used as the name of a Vala, properly the place where he dwells (the Halls of Mandos), whereas his real name is Námo_ (WJ:402). _In Tolkiens mythology, the "Halls of Mandos" are the abode of the dead, where their spirits remain until they are released from this world (in the case of mortals) or rebodied (in the case of Elves except for those who are refused or themselves refuse further incarnate life, and so remain in Mandos indefinitely). In the Etymologies, Mandos (also Mandossë) is interpreted somewhat differently, "Dread Imprisoner" (MBAD (MANAD),VT45:32) or in a deleted version "Dread Doom" (VT45:33, where Mandos was asigned the stem Mandosse-). The interpretation "Dread Imprisoner" would suggest that Tolkien at the time thought of Mandos as being also properly the name of a person, the Vala Námo, not the name of a place. See also Mando.