旋踵: [书面语] in the brief time it takes to turn round on one's heel -- in an instant; immediately 短语和例子旋踵即逝 vanish before one has time to turn round; disappear in the twinkling of an eye
不旋踵: [书面语] (来不及转身; 形容时间极短) in less time than it takes to turn one's heels; a very brief moment
