Creepingly is a word that describes something that is advancing in a slow, gradual, and unnoticeable way. Synonyms for this word include stealthily, gradually, slowly, insidiously, imperceptibly, and almost imperceptibly. Stealthily implies a secretive and sneaky approach, while gradually emphasizes a steady and slow progression over time. Insidiously suggests a harmful and treacherous advance, and imperceptibly indicates that something is happening without being noticed or detected. Other synonyms for creepingly include subtly, cautiously, and quietly, all of which denote a careful and subtle approach to something. In essence, any word that conveys the idea of a slow and stealthy progression can be considered a synonym for "creepingly".