Peace
Security
noun, な adjective
Relax in peace. Wherever you are that's allowing you to do this, it is certainly peace. You feel security too.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
あの忌まわしい家族が未来永劫安泰でいられるなんて、誰が信じたいと思う？
Who would want to believe that horrible family would have eternal peace?