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

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