Definition of "act up" in English

Verb

1. To misbehave; to cause trouble.

   • My allergies act up during hay fever season.
   • Children may act up in class in an effort to get attention.
   • My car started acting up after I drove through that huge puddle.

2. To perform the duties of an appointment senior to that actually held; to deputise for a superior.

   • Acting up is always recommended when you find yourself under a new boss.