Looking under skirts, or down shirts to get a peak of a naughty part.
While on the escalator, Fundo used his camera phone to get a photographic sneak peak under the skirt of the female standing 3 steps ahead of him. Fortunately for him, she wasn't wearing panties.
To get high in secret. Not to be confused with "sneak peek", a common expression used to describe a preview of a product.
I got a sneak peak in on my lunch break.