on the spur of the momentexprexpression: Prepositional phrase, adverbial phrase, or other phrase or expression--for example, "behind the times," "on your own."
(spontaneously)
一时冲动 yì shí chōng dòng
不假思索 bù jiǎ sī suǒ
心血来潮 xīn xuè lái cháo
We decided to go to Las Vegas on the spur of the moment.