In forming the feminine with the affix ṅīp , au is optionally the substitute of the final manu (as well as the udātta ai).,
The phrases ऐ and उदात्त are understood. The force of वा is to make the substitution optional. Thus we have three forms, मनायी or मनावी or मनुः, all meaning 'the wife of Manu'. The word मनु has udatta accent on the first vowel, as it is formed from मन् by the affix उ (Un l. 10), which by Un I. 9 is treated as having an indicatory म्, and affixes having indicatory न् throw the accent on the first syllable. (VI. I. 197),