Definition from Wiktionary, the free dictionary
- (intransitive, idiomatic) 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.
- (intransitive, idiomatic) To perform the duties of an appointment senior to that actually held; to deputise for a superior.
- (misbehave): carry on