When a person decides to do something, or go somewhere, without any planning, it is said to be a ‘spur of the moment’ decision. To think of this expression literally, try understanding it like this: A moment is an instant in time. A spur is something small which grows from something that is larger.
心血來潮，我去理了個頭。
On the spur of the moment (作副詞片語用) 指事先沒有計畫，卻突然想到就做。本句的 a decision 用 spur of the moment 加以形容，就是「一個衝動的決定」。