The timeline for this entry shows the time when the name Mar-nu-Falmar would meaningfully be used, starting with the time of the Downfall itself near the end of the Second Age
'Home under the waves', a name of Númenor after its destruction, used among the exiles of that land in Middle-earth.
Notes
1
This is the direct translation given by Christopher Tolkien in his Index to The Silmarillion. To put this interpretation in context, Elvishmar carries a sense of 'home' or 'dwelling-place', and its use here for 'land' implies something like 'homeland', referring to the lost home of the exiledDúnedain.