Definition

Verb (intransitive): 1. To malfunction or operate in an undesirable, often painful, recurring way. Typically used for health conditions or mechanical objects. 2. To behave badly, disruptively, or in a silly, improper manner.

Usage

This phrasal verb is intransitive and does not take a direct object. It is often used in informal contexts to describe problematic behavior or recurring issues.

Examples

• For recurring pain/malfunction:
  • My old knee injury acts up whenever it rains.
  • The car's engine has started to act up; we should get it checked.
• For misbehavior:
  • The students always act up when there's a substitute teacher.
  • He acted up at the restaurant, so we had to leave early.

Advanced Usage

• The phrase can imply a temporary, episodic problem rather than a constant state. Something that "acts up" is not always problematic, but becomes so at specific times.
• It can be used figuratively for non-physical things: "My computer acts up if I have too many programs open."

Variants and Related Words

• Act out (phrasal verb): Often overlaps with the "misbehave" meaning of , but can also specifically mean to express emotions or conflicts through behavior, or to perform a scene.
  • The child acted out his anger. (expressed through behavior)
  • The actors acted out the famous battle. (performed)

Synonyms

• For malfunction/recur: flare up, trouble, malfunction, play up (UK informal).
• For misbehave: misbehave, cause trouble, be disruptive, fool around, clown around.

Related Phrasal Verbs

• Play up (chiefly UK): A close synonym for both meanings of .
  • My back is playing up again. (recurring pain)
  • The children were playing up all afternoon. (misbehaving)

Related Idioms

• To be on one's best behavior: The opposite of . To behave very well.
  • You must be on your best behavior at the wedding.

Verb

1. make itself felt as a recurring pain
   • My arthritis is acting up again
2. misbehave badly; act in a silly or improper way
   • The children acted up when they were not bored