To nosh is to have a snack. You might nosh on a slice of cold pizza and a handful of chocolate chips after school.
Many people like to nosh on popcorn while watching movies, and you could nosh on the cookies you baked for the school bake sale. You can also call a meal a nosh, especially if it's just a snack: "Let's have a nosh before we head back out on the hiking trail." Nosh comes from the Yiddish word -nashn, "nibble," and its earliest use in English, around 1917, was as a shortened form of nosh-house, or "restaurant."