Antonyms for the expression "more back-slapping" could include terms such as reserved, unemotional, unenthusiastic, and unsociable. These words convey a sense of detachment or introverted behavior, rather than the overt praise and camaraderie that might accompany back-slapping. Other antonyms might express negative emotions, such as hostility, animosity, or conflict, which would run counter to the cheery, pro-social tone of back-slapping. Ultimately, the choice of antonyms depends on the context in which the phrase is being used, and the desired effect or impression that the writer or speaker wishes to convey.