The phrase "most slap happy" is used to describe someone who is overly jolly or excitable, often to the point of being irritating or disruptive. Its antonym, therefore, would refer to someone who is calm, composed, or level-headed. Words like "serious," "sober," or "unemotional" could all qualify as antonyms for "most slap happy." Other antonyms for this phrase could include "stoic," "reserved," "mature," and "dignified." These words all describe individuals who are unlikely to be excessively expressive or exuberant, opting instead for a more restrained, thoughtful approach to their actions and emotions.