"Admit it, admit you're the Adept! Admit it!" "I'm not adept, I'm adopted," said the girl. "I'm sure not adept at anything, can't you tell that?" (5.17.18-19)
This miscommunication is funny in a dark sort of way, given that Elphaba is terrorizing Dorothy here. Dorothy understands the word "adept" as meaning skilled, while Elphaba is using it in the way Madame Morrible once did: as a type of apprentice or trainee. It's interesting that Dorothy brings a similar-sounding word to the mix, "adopted," given how significant a theme family is in the book.