To act up (or be acting up) is an English phrasal verb that means to behave badly, malfunction, or become painfully active. Depending on the subject, it applies in three primary ways:
- People (Especially Children): To misbehave, cause trouble, or throw a tantrum (e.g., “The kids always start acting up when they get tired.”).
- Machinery, Electronics, & Vehicles: To malfunction, operate erratically, or fail to work properly (e.g., “My laptop has been acting up all morning.”).
- Medical Conditions & Physical Ailments: To flare up, become active, or cause pain or discomfort (e.g., “My knee acts up whenever it rains.”).
In this phrasal verb, act refers to behavior or performance, while up serves as an intensifier denoting abnormal or negative disruption.
Movie and Television Citations
In film and television, screenwriters frequently use “acting up” across emotional family dramas, comedies, and westerns to describe troublesome behavior, broken equipment, or recurring health issues:
- Hancock (2008): Ray (Jason Bateman) defends a troubled young boy by framing his disruptive behavior as an emotional reaction to his parents’ marital problems: “Michel is not a man. He’s a little boy, okay? And his parents happen to be going through a really bad divorce and that’s why he’s acting up.”
- Home Alone (1990): Kevin’s mother scolds him after a chaotic dinner argument, singling him out as the sole troublemaker in the household: “Fifteen people in this house, and only you have to make trouble.” | “I’m getting dumped on.” | “You’re the only one acting up. Now get upstairs.”
- Dark Shadows (2012): Barnabas Collins is picked up in an unreliable, poorly maintained vehicle whose owner refuses to pay for repairs: “Sorry I didn’t grab you down at the station. Damn wagon’s been acting up. Miss Elizabeth won’t spend the money to get her fixed.”
- Rango (2011): the local lawman rushes off to handle a familiar outlaw known for causing town disturbance: “Sheriff, where are you headed?” | “There’s trouble down at Dry Creek. Bad Bill’s been acting up again.”
- The Goonies (1985): Chunk complains about his physical discomfort and allergies while navigating dusty underground tunnels: “It’s dusty, my hay fever’s acting up, and you always break something.”
Sentence Examples
Behavior & Misbehavior
- “If you children keep acting up, we aren’t going to the amusement park.”
- “I don’t know what’s wrong with my dog; he never acts up like this unless he’s sick.”
Machinery & Electronics
- “My computer acted up yesterday, so I couldn’t finish my report on time.”
- “The car is acting up again, so I’m going to have to take a cab to work today.”
Medical & Physical Conditions
- “Can you open this jar for me? My arthritis is acting up.”
- “When my allergies act up, I usually take one pill, but today was definitely a two-pill day!”
More Phrasal Verbs with Act
- Act Out: To express repressed emotions or desires through disruptive behavior, or to perform a story or role in a dramatic fashion.
- Act On (or Act Upon): To take action in response to information, advice, or an impulse.
- Act Up To: To equal, match, or live up to a standard or expectation.
- Act For: To represent someone or act as their legal or official agent.