Definition of act up
intransitive verb
1
:to act in a way different from that which is normal or expected: such 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
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