Slap-happy is a term used to describe someone who is careless, reckless, or overly happy. Opposite to its meaning, there are several antonyms that can be used to describe someone who is serious, careful, and calm. These antonyms include somber, grave, sober, judicious, and prudent. These words all convey a sense of thoughtfulness and careful consideration which are the opposite of the impulsive behavior that slap-happy people display. Someone who is somber or grave will not be prone to sudden outbursts of joy or recklessness. Instead, they are likely to be reserved and cautious in their actions.