-ու

Etymology 1
From.

Etymology 3
From.

Etymology 1
From, the feminine form of the participle suffix. See also.

Etymology 2
Contamination of stem vowel with the,  suffix.

Etymology 3
According to Godel, identical with the stem vowel. The remaining stem, standing in contrast to the nominative/accusative singular (which lost its stem), was apt to function as a full-fledged case form.