noun, na-adjective, noun which may take the genitive case particle 'no'
world peace; peace throughout the land
Literary or formal expression for a state of universal peace, often used in historical or idealistic contexts. As a yojijukugo, it carries a classical, somewhat grand tone.
江戸時代は天下泰平の世の中だったと言われる。
The Edo period is said to have been an era of peace throughout the land.
天下泰平を願う気持ちは今も昔も変わらない。
The wish for world peace remains the same now as in the past.
na-adjective, noun which may take the genitive case particle 'no', noun
Describes a person's easygoing, untroubled state of mind, often implying a lack of awareness of real problems. Can be used humorously or critically. More colloquial than sense 1.
彼はいつも天下泰平な顔をしている。
He always has a carefree look on his face.
試験前なのに天下泰平で遊んでばかりいる。
Even though exams are coming up, he's totally relaxed and just playing around.