From a bio.: He liked etymology and unusual facts about words.
web page

Also probably no help:
AGID database

There are several ref.'s to it having to do with disease; not the likely meaning here, I think!

Best guess, and it's not a good one, is that he coined the word from parts of others. I first tried a-, but had no credible results for -gid. I suppose -id could refer to that part of our psyches, but the ag- prefix doesn't make a whole lot of sense. In this particular ref., when I looked up ag- I was referred to ad-, which shows these meanings:
ad-
prefix

Definitions:

1. to, toward
adsorb
advance

2. near
adrenal
[< Latin ad "toward, near" < Indo-European]
encarta