What is Cheeksneak?
When a girl is wearing a skirt or shorts that reveal the under-curve of her asscheek she is offering a cheeksneak.
When a girl moves or walks a certain way and her clothing accidentally reveals her ass; a sneak-peek at her cheek.
Similar to a nipslip only for asses.
"Those shorts give a nice cheeksneak on that girl."
"Cheeksneak is the new sideboob."