>gändr - "great serpent", see also Jörmundgandr, the
>World-Serpent (literally Jör, he who encircles; mund,
>the world; gandr, great serpent); can refer specifically to one
>of the greater dragons, as with the English wyrm; also modifies
>én to some degree in its Draconic usage, which may account
>in-story for the seemingly extraneous umlaut.

This is only in relation to 'real' norse. If this is what it means in UF, then fine and dandy. But, Jörmungandr in non-UF norse is made up of two separate words. Jormun/Jörmun, which means great. And Gandr, which means a few diffrent things, depending. It would mean stick, especially a staff for rituals. Or it could mean wolf. So, most probably, Jörmungandr means 'great staff', possibly due to the long narrow shape.

This as an aside, what it all means in UF is up to the authors writing what we enjoy reading.