Definition of sneak up phrasal verb from the Oxford Advanced American Dictionary

 

sneak up

 phrasal verb
phrasal verb
 
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Phrasal Verbs

sneak up (on somebody/something)

 
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to move toward someone very quietly so that they do not see or hear you until you reach them He sneaked up on his sister and shouted “Boo!”
See the Oxford Advanced Learner's Dictionary entry: sneak up