Seemingly a verb used in the West-Coast of Scotland (the town of Paisley, for example) that refers to closing something, most often some sort of door or window. Does not seem to be in common useage with younger people, judging by past definitions on this site.
When the turtle head poops out, but is not fully dislodged. As your bum checks are nice and smoothe, the full poop is in tack. However, in a sudden panic, your bum cheeks squash together fastly and snib (break) the poop which in turn releases and falls south. Half still inside ya and the other half at bottom of your leg.