Allison, I had been wondering about that myself and have also been googling a bit.

It seems that the Watersons haven't the faintest ideea what "Residue" is supposed to mean in that contect, so it's almost certainly a corruption or mishearing of "Levy Dew".

Apart from the possible meanings of "Levy Dew" given above, another one is that it may come from "Levedy" which was Old English for "lady". So we have suggested etymologies from French, Welsh, or Old English - take your pick.

And have a good one, anyway!

Marje