Acting up is a term used to describe mischievous or naughty behavior. It can include anything from minor pranks to more serious misbehavior. NAUGHTINESS (11 letters) is a synonym for acting up and is often used to describe children's misbehavior. MISBEHAVING (11 letters) is another word for acting up and is used to describe any kind of inappropriate behavior. ACHY (4 letters) is an acronym for Acting Childishly Hurts Yourself, which is a reminder that acting up can have negative consequences. Horsing around (13 letters) is a phrase used to describe playful, but potentially dangerous, behavior. All of these terms are related to the concept of acting up and can be used to describe mischievous or naughty behavior.