Slap happy is a phrase that refers to being wildly or foolishly happy. It is often used in a sarcastic or mocking way to describe someone who is acting silly or immature. Some synonyms for slap happy include giddy, giggly, frivolous, and inane. A person who is slap happy may also be described as deliriously happy, ecstatic, euphoric or overjoyed. This type of mood can be brought on by a variety of things, such as winning a game, getting a promotion, or receiving good news. It's important to remember that being slap happy shouldn't be confused with being careless or thoughtless, nor should it be used to make fun of someone's genuine happiness.