The phrase "hadst bone to pick" is an archaic way of saying "you had a problem to solve". The spelling of "hadst" is based on its pronunciation, which is /hædst/. The "st" at the end of the word indicates that it's in the second person singular form of the past tense, used in Early Modern English. The phrase "bone to pick" means to have an issue with something or want to discuss a problem, and it dates back to the 16th century.