Intriguingly though, uman remains an etymological mystery. How can it mean "at one time"?

+Björn Fromén perhaps it was invented on the spot without any precedent

+Tamas Ferencz Even so, to create a word without any recognizable connexion seems very uncharacteristic of Tolkien. If uman had meant 'at no time', I'd have been less surprised.

+Björn Fromén
I was toying with the idea of them being the other way round, uman meaning "fact", and enwa meaning "at that time" (en- can be found in time/space related words), but enwa fits "fact, reality" too neatly, and wouldn't make it easier to explain why uman meant 'fact'.

+Björn Fromén
although, if you think of it, we have úlume attested in PE17 "ever; at all time (in a series or period" where it is also difficult to explain ú - it does not seem to carry a negative meaning. Also ullume "forever" - I know peple interpreted this as 'not for ever' but the negation is already is already there in the sentence in úye.
With that in mind it is not inconceivable that the initial u in  uman is related to the same element meaning 'ever' - but it still does not explain -man.

+Tamas Ferencz
And nor does it explain why the sense is 'at ONE time'. I agree that that u- in ullume and úlume apparently means 'any' rather than 'not, un-'; and this could possibly apply to uman as well, if it were derived from the root variant UM(U)- and the meaning had been 'at any time' (cf. sinan 'at present' = *'at this time').

+Björn Fromén another question is whether uman refers only to the past, as in "once upon a time" (essentially the same as yallume), or can it refer to the future as well, e.g nahtuvanyel uman helde mányanten "one day I will kill you with my bare hands"?