Stink Thimble
When you use very thin sheets of toilet paper or misjudge the velocity at which you should be wiping and your finger goes through the paper and straight up into the brown rusty mess. Thus leaving a small coating of turd on the end of your finger which resembles a brown thimble
"Dammit Margaret", exclaimed George. "I wish you'd stop buying this cheap toilet paper from the corner shop, this is the third Stink Thimble I've given myself this week"