Could someone please let me know the technical term (that has slipped my
mind) for a word or term important unchanged from one language into
another (like tatpurusha in Pali). It is a good old Anglo-Saxon word I
think.

I am in the midst of preparing the notes for my retranslation of the
(Anuruddha) Upakkilesa Sutta (M 128), a beautiful sutta on spiritual
friendships amongst arhats.
The 3rd personal vocative plural, Anuruddhaa, is used by the Buddha to
address Anuruddha, Nandiya and Kimbila.

IB Horner translates the Pali "Anuruddhaa" literally as "Anuruddhas"
which is correct but sounds odd in English. Nyanamoli and Bodhi simply
render it as "Anuruddha" which addresses only Anuruddha. I thought it
would be better to simply use the original Pali: Anuruddhaa in the
translation (as a sort of tatpurusha).

Sukhi.

Piya