In Gathering Blue, what has Matt pasted to his chest?
The answer to this question can be found in Chapter Ten as Kira feels a strange premonition of some harm that will come to Matt if he is allowed to go on the hunt with the others. Because of this, she and Thomas go down to stop him going with the men. As they draw close to Matt, they discover the measures that he has gone to to make himself more "manly" and supposedly show that he is bigger than he actually is and therefore able to go on the hunt because of his "manly pelt":
The little boy raised both arms above his head to show her his joke. Kira looked. His underarms were thick with some kind of growth. "What is that?" she asked him. Then she wrinkled her nose. "It smells terrible!" She touched it, pulled some away, and began to laugh. "Matt, that's swamp grass. It's awful stuff. What do you mean, plastering yourself with it?" She could see that he'd smeared it on his chest as well.
Therefore we can see that, in his attempt to jokingly pass himself off as a man who is old enough to go on the hunt, Matt pastes smelly swamp grass to his chest. This of course makes Thomas and Kira take him to their rooms so that he can be properly cleaned and have a bath, which is something that Matt does not necessarily appreciate.