4 ideas
Józio, a thirty-year-old writer trying to find his mature self, is visited by Professor Pimko, who treats him as a child until he actually becomes one and marches him off to Piórkowski's school. From there he is lodged with the 'modern' Youthful family, whose schoolgirl Zuta he becomes infatuated with, and finally ends up at his aunt's gentry manor, where master-servant roles collapse into brawls.
The 'pupa' (behind or bum) is the diminished, infantile self that other people's gaze and treatment attach to you. Pimko does not argue with Józio. He addresses him with condescending tenderness, and that treatment alone reshapes him until he behaves like a child. Immaturity here is less a stage you grow out of than a condition that others can manufacture in you through how they relate to you.
No one has an authentic self prior to social interaction. Every person is squeezed into a 'Form' (a face, a manner, a style) by the people facing them, and they squeeze others in turn. Escaping one Form only lands you in another, so the only freedom available is awareness of this deformation, not release from it.
Look at any encounter as two people fitting 'mugs' (gęba) onto each other, fixed expressions and roles that each then has to act out. This makes visible how the school duel of grimaces, the progressives' performed modernity, and the manor's master-servant etiquette all work the same way. Each is a contest over who gets to impose the face the other must wear.