Definition from macmillandictionary.com
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The extra power stations are idle when demand is lower.
Valuable machinery is left to lie idle for long periods.
Many men were made idle as the mills closed down.
Get upstairs and wake up that idle brother of yours.
You’re just bone idle, the lot of you.
Don’t worry. I have not been idle while you were away.
This country will not remain idle if its friends are attacked.
It was only from idle curiosity that she opened the book.
It didn’t sound like an idle threat to me (=it was a real threat).