Back and fill is an idiomatic expression that means to vacillate or to change one's mind frequently on a particular topic or issue. There are a variety of synonyms that can be used to convey a similar meaning, such as dither, hesitate, waver, vacillate, falter, fluctuate, and oscillate. Other synonyms that can be used to describe indecisiveness or uncertainty include equivocate, hedge, second-guess, and prevaricate. It is helpful to have a variety of synonyms at your disposal, as they can express a slightly different nuance or emphasis depending on the context in which they are used.