WHAT IS IT?
A thing is not a thing. Nor is it a thing and it certainly isn't the Thing 
To clarify: The word thing has several different meanings depending on where you are and what your doing at the time.
To clarify still further: A thing is not a thing, a Norwegian Sweedish jazz band. Nor is it a thing-in-itself and it certainly isn't the Thing, Benjaman Grim of the Fantastic Four.
Now that we've got all that straightened out I can finally reveal that a thing was the governing assembly made up of the free people in the community and presided over by lawspeakers. It was held in a thingstead which, as far as my "exahustive" research can tell means "a place where the thing happens"
WHERE DO YOU GET IT?
It comes down to us from the lands of the Norse although over time it filtered through the Germanic tribes and was also introduced into some Celtic societies. In these modern times the thing lives on in the official names of national legislatures as well as political and judicial institutions in the Nordic countries.
USING IT IN REAL LIFE:
- Essentially the thing is a parliament at its most basic level. There are still hierarchies and the usual politics that you get with any gathering but on the whole it was a place for solving disputes and making laws.
A famous incident took place when Þorgnýr the Lawspeaker told the Swedish king Olof Skötkonung that the people, not the king, held power in Sweden; the king realized that he was powerless against the thing and gave in.
USING IT IN WRITING:
- As I've noted in a few other posts over the course of this marathon, anywhere you get people sooner or later you get politics and you can't have politics without plot  It may be that everyone smiles and nods at the concept of one man, one vote but many a closed door has hatched a devious scheme.
- The easiest situation to imagine is that your hero has a grievance against someone of much higher standing than him and the thing is the only place where he might be heard in relative safety. 
- Perhaps the lawspeaker is the baddie. What do the people do when he begins his grab for power protected, as he is, by the law of the land?
- Don't forget the magical element.  It might be that the magical races of the ancient land could hold a thing, or attend one, to air their grievances against a particular group of humans for some wrong either actual or imagined. 
MIGHT ONE TAKE OVER THE WORLD WITH IT?