on the spur of the moment [ɔn ðə spə: ɔv ðə ˈməumənt]

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一时冲动之下，当即

He asked her to marry him on the spur of the moment.他一时冲动之下向她求婚了。
She went to London on the spur of the moment.她一时兴起就到伦敦去了。