This is a noun of place, taken to refer to the womb, or the grave, among other opinions.

This is an active participle from the verb meaning ‘to be settled’ in a place.

Abū Jaʿfar al-Madanī

Ibn ʿĀmir al-Shāmī

وَهُوَ ٱلَّذِىٓ أَنشَأَكُم مِّن نَّفْسٍ وَٰحِدَةٍ فَمُسْتَقَرٌّ وَمُسْتَوْدَعٌ ۗ قَدْ فَصَّلْنَا ٱلْـَٔايَـٰتِ لِقَوْمٍ يَفْقَهُونَ