Sneak up is a commonly used phrase to describe a stealth approach or a sudden, unexpected appearance. However, there are a variety of synonyms available to describe this concept. Some alternatives include creep up, approach quietly, tiptoe, slip up, and approach furtively. Other synonyms for sneak up could be slowly moving towards a destination, moving stealthily closer to a subject, gaining ground without being noticed, or closing in on someone unnoticed. These synonyms are useful when trying to describe different scenarios where sneaking up could occur, such as in animal behavior, approaching a person to surprise them, or when trying to convey a sense of danger or vulnerability.