There are a number of synonyms that can be used to describe the phrase "on and on." One of the most common synonyms is "endlessly," which suggests something that seems to have no discernible end. Another phrase that can be used to describe "on and on" is "without pause," which implies that the action or event in question is continuing without interruption. "Continuously" is another synonym that can be used to describe "on and on," while "incessantly" implies a repetition or recurrence of something that may feel endless. Other similar phrases include "unendingly" and "eternally," both of which suggest a sense of timelessness or infinity.