sneak up (on somebody) = appear suddenly and scare somebody
- He sneaked up from behind and scared me to death!
- Don 't sneak up on people like that in the middle of the night.
- She wanted to surprise me and sneaked up on me from behind.
- Don 't sneak up. Give me a warning that you 're coming. Call or make noise.
- The children sneaked up behind us and started laughing.
- He sneaked up behind and shouted "surprise!"