Main Entry:

act up

Function:

intransitive verb

Date:

1903

1: to act in a way different from that which is normal or expected: as a: to behave in an unruly, recalcitrant, or capricious manner <the children were acting up> b: show off c: to function improperly <this typewriter is acting up again> 2: to become active or acute after being quiescent <her rheumatism started to act up>