Heill, Símon.

> The word SIGALDRY came up on CALL MY BLUFF recently.
> It's also mentioned on this site:
> http://www.suite101.com/article.cfm/tolkien/91315
> It's just that it seems to have an Icelandic ring about it - GALDUR, GALDURS
> = witchcraft, and I'm sure they said that SIGALDRY had a similar meaning.

The morphemes look like they could also be of Latin origin.

> Also, doesn't the prefix SÍ mean ETERNAL?

Something like that, yes. It can refer to repetition
as well as long duration.

> Can anyone shed any light on this ancient word from a 12th century English
> document, please?

I have no idea :) If it really is of ON origin one might also imagine
a compound sig-galdr; battle-magic (formed like sig-rúnar).

My gut feeling is that this is not an Icelandic word but
it doesn't seem completely impossible.

Kveðja,
Haukur