The term 'not at all serious' can be summarized by two crossword solutions: SILLY (5 letters) and FLIP (4 letters). SILLY is an adjective that means 'lacking in seriousness or maturity; foolish'. FLIP is an adjective that means 'lighthearted, carefree, and unconcerned'. Both words describe a lack of seriousness, suggesting that the term 'not at all serious' is used to describe something that is humorous or playful.