razor's edge
or razor edge
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used in various phrases to refer to a dangerous position or a position in which two different things are carefully balanced危险的困境；岌岌可危的处境 -
He's a thrill-seeker who likes living on the razor's edge.他是个追求刺激的人，喜欢身处险境。
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The country's future is balanced on a razor's edge.国家的未来处于危险的边缘。
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He walks a/the razor edge [=walks a/the fine line] between humor and bad taste in his comedy.他的喜剧表演就如同走钢丝，很容易从幽默跌落至庸俗。