When you peek, you take a quick glance at something. It's incredibly tempting when you're little to peek at your birthday presents, but you soon learn how disappointing it is to spoil the surprise.
A sneaky look at something is a peek. You might peek at someone if you're shy or you don't want them to catch you looking. You can also describe a glance at a part of something as "a peek" — like when you take a peek at a napping child. Peek comes from piken, a 14th century word meaning "look quickly and slyly." Until the 15th century, the word peek was used interchangeably with keek and peep.