Shelfing is an activity where people try to fit themselves inside a closet. It is a prerequisite for proper shelfing that the closet is not larger than your body volume plus 0,5 m3.
I saw two gals at the museum who had lots of fun shelfing in an antique closet, with the curator planking on the top of it.
If something is shellings, it's absolutely amazing. This is most used in UK Garage/ DnB subcultures. Many variations of this word is used to mean different things. Shellerz/ shellington / shelby are all different variations meaning the same thing. If a DJ is "shelling down" at a rave, it means they're shutting it down with their lyrical skill.