asuṅ (as) is substituted for the final of puṃs in the strong cases.,
The word पुंस् is derived from पा (to protect) + डुम्सुन् (Unadi IV. 178), the म् being changed to anusvara. So when स् of पुंस् is replaced by अस् we get the form पुमस्, the उ of असुङ् indicates that न् should be added in the strong cases after अ 7.1.70, so we have पुमान्, पुमांसौ, पुमांसः ॥
This substitution must take place in its incipient stage before the affixes are added, (उपदेशिवद्भावः) : otherwise the accent will be wrong. The compounds have acute on the final, 6.1.223, therefore, परमपुंस् has acute on पु. and in the Nominative Singular परमपुमान् the acute will remain on पु, but it is intended that it should be on मा, thus परमपुमान् ॥ The simple word पुमान् of course, has accent on पु ॥,