The phrase "most back slapping" denotes excessive and insincere praise or flattery. Its antonyms would therefore be words that convey the opposite meaning, such as "critical," "disapproving," "skeptical," and "doubtful." Antonyms for "most back slapping" could also include words such as "reserved," "quiet," "modest," and "understated," which convey a more restrained and genuine form of praise or admiration. In short, any word that reflects a more measured and thoughtful approach to praise could serve as an antonym for "most back slapping.