1. To peer, glimpse, or glance under (something) quickly, shyly, or inconspicuously. I peeped under the rug to see if my toddler had hidden anything there. Sarah was peeping under the curtains trying to find our cat.

2. To be revealed partially from underneath something. I hate the way my shirt peeps under my jacket like that! I tried to cover up the graffiti with black paint, but there was still a bit of color peeping under it.