ط م نṭ-m-n
— اِطْمَأَنَّ يَطْمَئِنُّ اِطْمِئْنَانًاiṭmaʾanna yaṭmaʾinnu iṭmiʾnānan (verb form (bāb) افعِلَالafʿilāl, augmented quadriliteral (rubāʿī mazīd)): for the heart to become tranquil and at rest; to settle.
مُطْمَئِنّmuṭmaʾinn pl. مُطْمَئِنَّةmuṭmaʾinnah (active participle, ism fāʿil): tranquil; settled. A morphological point: it is on the same pattern as اِشْمَأَزَّishmaʾazza and اِقْشَعَرَّiqshaʿarra — and this pattern carries the sense of a prevailing state. The lexicographers say that a metathesis (qalb makānī) has also been described in the original form طُمَأْنِيْنَةṭumaʾnīnah. 13 times in the Quran: ﴿أَلَا بِذِكْرِ اللّٰهِ تَطْمَئِنُّ الْقُلُوْبُ﴾ (Ar-Raʿd 13:28) and ﴿يٰۤأَيَّتُهَا النَّفْسُ الْمُطْمَئِنَّةُ﴾ (Al-Fajr 89:27).