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useless; in vain; aimlessly; idly
Note: adverbial or literary usage
いたずらに時を過ごすべきではない。
We should not let time pass idly.
いたずらに不安をあおる報道は避けたい。
We should avoid reporting that needlessly stirs up anxiety.
いたずらに争いを広げるつもりはない。
I have no intention of widening the conflict for no reason.