This is the 6389th most frequent Japanese word.
シレ
"シレ" does not have a standard definition in Japanese; it could be a fragment, onomatopoeia, or require context for interpretation.
The word 'シレっと' is used here to describe an action done bluntly or unapologetically.
彼はシレっと自分の意見を言った。
He bluntly stated his opinion.
The word 'シレ' is used poetically to describe the serene and tranquil state of the sky.
その日の空はシレこうとしていた。
The sky on that day was calm and clear.
'シレっと' describes her manner in continuing the conversation without showing her internal feelings.