n. Named for the U.S. Army Rangers, an elite group of soldiers.
Commonly done as a military initiation joke, the subject is blindfolded, and two people hold his shoulders down. Another soldier (more commonly one who has been in the service for a while) stands with his pants down so that when the other two release the subject, his face plants right in the standing soldier's ass-crack.
"Man, my buddies thought it would be cool to have me do Ranger Sit-ups. Even the sarge was in on it..."
"Alright, private. Time to try your luck at Ranger Sit-ups."