act up
Proposed by a contributor, not yet supported by evidence.
Meaning · English
behave badly or mischievously
Literally: perform in a way that is not normal
Where it’s used:
Used
Widespread English
Sources & notes
Listed in Wiktionary’s “English idioms” category (retrieved 2026-07-29); definition pending editorial review. Definition drafted by AI (@cf/meta/llama-3.3-70b-instruct-fp8-fast) and auto-approved by rule on 2026-07-30; pending editor spot-check.
Common questions
What does “act up” mean?
behave badly or mischievously Literally: perform in a way that is not normal
Is “act up” formal or informal?
No register label has been reviewed yet — treat it as neutral and check the usage notes above.