首鼠两端

shǒushǔliǎngduān

[can't decide which to follow; be undecided in course of action like a rat's head looking two ways] 形容迟疑不决、 瞻前顾后

像吴三桂那样首鼠两端的人, 在初对于自成本有归顺之心, 只是尚在踌躇观望而已。 --郭沫若《甲申三百年祭》