act up

Concise Oxford English Dictionary © 2008 Oxford University Press:
act/akt/

▶verb

• 1 take action; do something.

  ■ (act up) informal behave badly.

• 2 (act for/on behalf of) represent on a contractual or legal basis.

  ■ (as adj. acting) temporarily doing the duties of another.

• 3 take effect or have a particular effect.

• 4 perform a fictional role in a play or film.

  ■ behave so as to appear to be: I acted dumb.

  ■ (act something out) perform a narrative as if it were a play.

▶noun

• 1 a thing done.

• 2 a simulation or pretence.

  ■ a particular type of behaviour or routine: he did his Sir Galahad act.

• 3 Law a written ordinance of Parliament, Congress, etc.

• 4 dated a record of the decisions or proceedings of a committee or an academic body.

• 5 a main division of a play, ballet, or opera.

• 6 a set performance: her one-woman poetry act.

– phrases
act of God an instance of uncontrollable natural forces in operation.
act of grace a privilege or concession that cannot be claimed as a right.
get (or be) in on the act informal become (or be) involved in a particular activity, in order to gain an advantage.

– derivatives
actability noun,
actable adjective,
acting noun.

– origin ME: from L. actus ‘event, thing done’, from act-, agere ‘do, act’, reinforced by Fr. acte.