The last fart you rip before you hit solid or liquid release (like pulling the jenga log and the tower falls). The fart that precedes having to take a dump - the one that is holding in the turd.
You're drinking with your buddies and ripping a good string of farts and after one real good one your realize that the next thing that will come out is crap. You proudly declare JENGA and everyone knows that you just pulled the last fart before the pile of turd drops out & you haul ass to the toilet.
I came up with this when pheasant hunting in KS with buddies. Walking through a field and no toilet in sight for miles and I shout out JENGA and head for a stand of trees to drop trou and take a dump. Since then the use of JENGA has spread a fair amount throughout the midwest. Help me pass it on.