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>