To treat something carelessly or to not take something seriously.
掉摆动；轻轻率。对事情采取轻率的漫不经心的态度。
唐·柳宗元《答韦中立论师道书》故吾每为文章，未尝敢以轻心掉之。”
He handled the safety inspection with a casual attitude, but this is not something to take lightly.
Learn the next connected idiom: 掉以轻心 (diào yǐ qīng xīn) → 心有余悸 (xīn yǒu yú jì)
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