Used in jails and prisons, to "check in" means to request a transfer into protective custody. Protective custody is an area within a prison or jail where security is tighter and officer presence is more pronounced. Protective custody is used mainly for rats, people with mental illness and sex offenders.

Straight-Up Whipped!!!

"Checking" simply refers to indicating one's interest in acquiring something or seeing what it is all about. On hip-hop forums, "checking" is usually expressed in "im/im not checking for it" in terms of being interested in downloading and listening for the latest album from a particular artist. If you dislike a particular artist, you can let everyone know how disinterested you are by saying "im not checking for it" to indicate that even though you can download the album for free, its not even worth your time

When someone "calls someone out" or makes fun of someone else. Good checks are funny. Usually when someone is checked, other people will laugh and sometimes join in to the "checking games"

talking about someone in a bad way

Check this out, look here, see this. Often used in a situation concerning something considered awesome. Also a skateboard stereotype slang word. It is most often used by guys. It can also be used concerning a pretty girl, as seen in example two.

Checkered or two-tone clothing items and accessories, particularly in context of Ska music, and the Ska scene.