The term 'not be serious' can be represented by four different crossword solutions: FOOL (4 letters), JEST (4 letters), KID (3 letters), and kidaround (9 letters). A FOOL is someone who acts foolishly or unwisely, often for comedic effect. A JEST is a joke or witty remark. To KID around is to act in a playful or silly manner. A KID is a young person or child. All of these words suggest that the phrase 'not be serious' implies a lack of seriousness or a lighthearted attitude.