Completely; entirely — used when something is gone or finished suddenly or fully.
風邪がけろっと治った。
My cold completely went away.
薬を飲んだら痛みがけろっと消えた。
After taking the medicine, the pain disappeared completely.
彼は約束をけろっと忘れていた。
He had completely forgotten the appointment.
Nonchalantly; as if nothing happened — showing a calm or unfazed attitude despite circumstances.
失恋したのに彼はけろっとしている。
Even though he had a breakup, he acts nonchalant.
先生に叱られたが、彼女はけろっとしていた。
She was unfazed even though the teacher scolded her.
ミスをしてもけろっとしている人は周りを驚かせる。
People who remain nonchalant even after making a mistake surprise those around them.