intransitive verb
1. : to act in a way different from that which is natural, normal, usual, or expected: as
a. : to behave in an unruly, recalcitrant, or capricious manner
skittish and inclined to act up with an unaccustomed rider
occasionally the river acts up
I'd advise you not to act up with me
b. : to show off
children acting up for the benefit of anyone who cared to notice
c. : to function improperly
this typewriter is acting up again
2. : to become active or acute (as of a physical infirmity) after being quiescent
an old injury to his right forefoot began to act up again — New Yorker