A descriptive adjective to be used if one's appearance or actions either resemble or call to mind the crevice between the cheeks of the ass.
Her smile looked like a buttcrack on it's side. OR... You are being such a buttcrack right now because you won't go out with me.
by Glutius Maximus April 18, 2005
V: to use your hands to open each side of your gluteus maximus to expose the rectum, then sit on an object while making sure that the rectum touches the desired object.
Rodney butt cracked Chris's pillow to get him back for that prank he pulled earlier.
by Richard Charles June 17, 2005