An adverb meaning to act indifferently or nonchalantly—showing little or no emotion or reaction.
彼は叱られても、しらっとした顔で席に戻った。
Even after being scolded, he returned to his seat with a nonchalant expression.
友達に借りた本を返すとき、彼女はしらっとしていて一言も謝らなかった。
When returning the book she borrowed from a friend, she was totally indifferent and didn't apologize at all.
問題点を指摘したが、上司はしらっと流してしまった。
I pointed out the problems, but the boss just brushed them off nonchalantly.