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sneak up (on someone or something)
To approach someone or something in a sneaky, furtive manner so as not to be noticed. Don't sneak up on me like that—you frightened the life out of me! We don't want the guards to see us, so we'll need to sneak up from the back.
1. To move or operate furtively or surreptitiously toward someone: The thief sneaked up behind the tourists and stole their luggage.
2. sneak up on To approach suddenly and surprisingly: The first day of spring sneaked up on me and I still hadn't gone skiing yet. Don't sneak up on me like that!