Islamic concepts

Ṭumāʾnīnah — peace of mind, (internal) tranquility, stillness, heart's ease

طُمَأْنينة

Muṭmaʾinn having peace of mind, contentment, quiet, tranquil, still, at rest, unruffled, at ease :)

مُطْمَئِنّ مُطْمَئِنّ

means both -ed and -ing (comforted, comforting)

http://corpus.quran.com/qurandictionary.jsp?q=Tmn