1. phrasal verb
If someone sneaks up on you, they try and approach you without being seen or heard, perhaps to surprise you or do you harm.
[V P P n] ⇒ I managed to sneak up on him when you knocked on the door.
2. phrasal verb → sneak If something sneaks up on you, it happens or occurs when you are not expecting it.
[V P P n] ⇒ Sometimes our expectations sneak up on us unawares.
COBUILD Advanced English Dictionary. Copyright © HarperCollins Publishers