The word "iminutive" is used in Yiddish, and, apparently, Bavarian grammar to refer to the second diminutive (i.e., of nouns).
The etymology of "diminutive" is clear. As for the provenance of the term "iminutive"--I have a sneaking suspicion that there was no term until one fine day it was divined that the concept of double-diminuition could be illustrated by example (i.e., diminishing a "diminutive" word), collaterally producing--what else but a brand new name for the concept?
When was the term first used? Who deserves credit for this oh-so-clever novellum?