diriment = absolute

I found that reference as well, Wordwind. At least, I think it's the same reference:

Diriment \Dir"i*ment\, a. [L. dirimens, p. pr. of dirimere. See Dirempt.] (Law) Absolute

I've never seen the term used in law before, but I assume that it is used in the sense of "absolute nullification". Since "nullification" nullifies something absolutely, the word "absolute" would appear to be unnecessary.

But jurists are adroit in finding escape hatches in words where no-one else [in particular, lawmakers] can find them. They have conjured up nullities which can be reversed, and other nullities, namely, "absolute nullities", which cannot be reversed.

"Nullities have been divided into absolute and relative. Absolute nullities are those which may be insisted upon by any one having an interest in rendering the act, deed or writing null, even by the public authorities, as a second marriage while the former was in full force. Everything fraudulent is null and void.

Relative nullities can be invoked only by those in whose favor the law has been established, and, in fact, such power is less a nullity of the act than a faculty which one or more persons have to oppose the validity of the act.

http://www.lectlaw.com/def2/n082.htm

BTW the same head scratching goes in law with "void" ["void ab initio"] and "voidable".

VOID - Having no legal effect or consequence.
http://www.lectlaw.com/def2/u041.htm

VOIDABLE - Having no legal effect or consequence if the party affected so chooses.
http://www.lectlaw.com/def2/u043.htm