The phrase "becoming stale" refers to something that is losing its freshness or appeal. To describe this state, there are several synonyms that can be used. For instance, the phrase "growing old" can be used to describe the diminishing quality of something that was once new and fresh. "Losing its spark" can also be used to describe the same phenomenon. Additionally, the phrase "getting tired" can be used to suggest that something has become less interesting and is losing its appeal. Other synonyms that can be used to describe the state of becoming stale include "losing its luster," "falling out of favor," and "going off".