When a really long turd protrudes from the butt. The turd is not released; the tension on the end of the turd exerted by the sphincter is enough so that the turd maintains its protrusion without breaking off or being retracted. This creates a silhouette of an erection, where the turd resembles the shaft and the buttcheeks resemble the balls. A butt boner.
"Check out this long dry one, bro! A perfect butt boner!"