The term 'not to be taken lightly' can be summarized by two crossword solutions: 'SERIOUS' and 'GRAVE'. Both words imply a sense of gravity and importance, suggesting that the matter at hand should not be taken lightly. 'SERIOUS' implies a sense of earnestness and sincerity, while 'GRAVE' implies a sense of solemnity and gravity. Both words suggest that the matter should be taken seriously and with due consideration.