The concept is very easy. Look at the example below for more detail...

1st person types 1st thing that comes to mind (troll)
2nd person types 1st thing that comes to mind when the word troll pops up (idiot)
3rd person types 1st thing that comes to mind when the word idiot pops up (brainless)
And so on, so forth.

There are no rules. Just try to limit it to one word.

And the first word I'm thinking of is:
Depression

We already have a thread series similar to this on Random. I think it's called "When I Say __, What's the First Thing That Comes to Mind?"