Nonchalantly; behaving calmly or as if nothing happened—used to describe acting with a composed, unconcerned demeanor.
彼は失敗を指摘されても、しれっとしているだけだった。
Even when his mistake was pointed out, he just remained nonchalant.
財布を見つけたけど、しれっと誰にも言わなかった。
I found a wallet but I calmly didn't tell anyone.
彼女はシレッと嘘をついて、その後も普通に話していた。
She told a lie with a straight face and continued talking as usual.
問題が起きても、しれっと振る舞う人には注意が必要だ。
You should be wary of people who act nonchalantly even when problems arise.