act up

present singular third person acts up · participle present acting up · participle past acted up · past acted up

1. idiomaticintransitiveTo misbehave; to cause trouble.

   My allergies act up during hay fever season.

   2 more examples

   Children may act up in class in an effort to get attention.

   My car started acting up after I drove through that huge puddle.

   Synonyms: carry on, act out, play up

2. idiomaticintransitiveTo 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.

   1 more example

   A few months through my contract, my clinical supervisor said that a registrar was leaving the post early for a research job, and asked if I’d be interested in acting up on the rota.

   2017, Adam Kay, This Is Going to Hurt: Secret Diaries of a Junior Doctor