To act up is to behave badly, or to stop working properly. The children act up, the printer is acting up.
It works for people and machines alike, and for machines it means intermittently rather than completely. Of a child it is milder than misbehave and slightly indulgent.
1phraseact upTo behave badly, or to stop working properly.
“The knee will act up if he walks too far.”
act up is a phrase.
Practised in Phrasal Verbs