If you're looking for synonyms for the word "mustiest," then you're probably talking about something that hasn't been touched or cleaned in a long time and has become stale or damp. Some words that could be used in place of "mustiest" include "foul," "moldy," "dank," "stale," "muggy," "humid," "soggy," "damp," "rank," "putrid," and "offensive." These words all convey a sense of something that is old, neglected, and possibly even unhealthy. If you're trying to describe a space or object that feels abandoned or neglected, any of these synonyms could be used in place of "mustiest".