The term 'more sullen' can be described by four different words, all of which have six to eight letters. The first word is 'dourer, ' which means having a more serious or stern expression. The second word is 'grumpier, ' which means having a more irritable or bad-tempered attitude. The third word is 'moodier, ' which means having a more unpredictable or changeable temperament. Finally, the fourth word is 'poutier, ' which means having a more sulky or petulant demeanor. All four words describe someone who is more sullen than usual.