english
1
general::
Phrase(s): act up [for a thing or a person] to behave badly. • This car is acting up again.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
1. Misbehave. For example, With an inexperienced rider, this horse always acts up. [c. 1900] 2. Malfunction, as in I'm not sure what's wrong with my car, but the transmission is acting up. In both usages up means "abnormally."
American Heritage Idioms