掉以轻心
Make it lightly.
Make light of.
Lower one's guard.

eg：It's an important job，you must not take it lightly.

中英俚语俗语来源：柳宗元·论师道书：“故吾每为文章，未尝敢以轻心掉之。”

同义词：毫不在意 To make nothing of.
反义词：严阵以待 Poised for action.