The phrase 'I don't think so' is often used to express disagreement or disbelief. The crossword solutions for this phrase include HUMPH (5 letters), HARDLY (6 letters), NAH (3 letters), NOT (3 letters), NOPE (4 letters), ASIF (4 letters), UHUH (4 letters), NOWAY (5 letters), LETSNOT (7 letters), NOWAYJOSE (9 letters), and FORGETABOUTIT (13 letters). All of these words are used to express a negative sentiment, and can be used in place of the phrase 'I don't think so'.