used to say that a decision, action, etc. is sudden and done without any planning

We hadn't planned to go away - it was one of those spur-of-the-moment decisions. 我们本来没打算走的——这是一时冲动作出的决定。

We just jumped in a car on the spur of the moment and drove to the seaside. 我们一时心血来潮，当即跳上汽车向海边开去。