The word "monishing" generally means to warn or advise someone sternly. Its antonyms, on the other hand, imply different meanings that are divergent. "Praise" is a good opposite to "monishing" as it means to express approval or admiration for someone's achievements or qualities. "Compliment" is another antonym that means to offer admiration, flattery or respect. "Encourage" is also an antonym that is similar in that it means to give support, hope or confidence to someone. More opposite words for "monishing" include "approve", "compliment", "laud", "flatter", "commend", and "applaud", among others.