木の役 学芸会の木の役 木G 木G(けいおん!)
Literally a costume in the shape of a tree. A frequent comedy trope in both anime and Western cartoons, the tree costume is often given to a character who really wants to be in an in-universe stage play but isn't considered a good enough actor for a proper role -- after all, a tree is supposed to merely stand in place and not say anything.
tree costume