Miss Trunchbull is the terrifying and ferocious headmistress of the school that Matilda attends. She is also the antagonist of the story. She is described in the story as "formidable," and as a "fierce tyrannical monster who frighten(s) the life out of the pupils."
The pupils are scared of Miss Trunchbull because she always "plough(s) right ... through them like a tank," and because she "insists upon very strict discipline throughout the school." One way in which Miss Trunchbull maintains discipline in the school is by locking naughty children inside a "tall but very narrow cupboard," with "bits of broken glass sticking out" from the walls, and "thousands of sharp spiky nails" on the inside of the door. This notorious cupboard is commonly known to the pupils as "The Chokey."
Miss Trunchbull is also known to throw children out of windows and across playgrounds. She throws a boy named Julius Rottwinkle out of a window for eating Liquorice Allsorts, and she throws a girl named Amanda Thripp "over the wire fence of the playground" because she doesn't like the girl's pigtails.
Elsewhere in the story, Miss Trunchbull forces one boy to eat an enormous cake in from the rest of the school, and another to "stand in the corner on one leg with (his) face to the wall." The children think of Miss Trunchbull as "an enormous bomb ... liable to explode and blow someone to bits any moment."
Miss Trunchbull obviously doesn't like children at all, and she takes great pleasure in terrifying and humiliating them at every opportunity. She freely admits that she doesn't like children, or "small people" as she prefers to call them. She says that "Small people should never be seen by anybody ... (and) should be kept out of sight in boxes like hairpins and buttons."