1630s, "of or befitting to a master or teacher or one qualified to speak with authority," from Medieval Latin magisterialis "of or pertaining to the office of magistrate, director, or teacher," from Late Latin magisterius "having authority of a magistrate," from magister "chief, director" (see master (n.)).
By 17c. often with a suggestion of "arrogant, imperious, domineering." Meaning "holding the office of a magistrate, proper to a magistrate" is from 1650s (see magistrate). Related: Magisterially.