a small piece of poop that is stuck in your ass
see dingleberryturtleheading and praire dogging can also be used as an insult in instances where you can't curse (i.e. in front of parents, little kids, etc)
omg you are such a effing butt nugget!
dude:ughh, i tried to take a shit today, but i still have a butt nugget
other dude:eww too much info!