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1. Misbehave badly; act in a silly or improper way
   "The children acted up when they were not bored";
   - carry on
    
2. Make itself felt as a recurring pain
   "My arthritis is acting up again"
    
3. [informal] Function erratically; stop working reliably
   - play up [Brit, informal]

Derived forms: acts up, acting up, acted up

See also: behave

Type of: ache, hurt, misbehave, misconduct, misdemean, smart

Encyclopedia: Act up