when you are ready to shit in a public bathroom, yet somebody comes in, and you hold it so that they won't hear embarassing noises coming from you (in essence, you are "sitting still"), and THEN you shit once they leave.
Roger: Hey, Dave! You don't look so good.
Dave: Man, someboy came in when I REALLY had to crap, and I had to have a shitstill until they left. It REALLY hurt to have to hold it for so long!