Green’s Dictionary of Slang

act n.

1. [late 19C+] (orig. US) a routine, a way of behaving, a performance.

2. [1940s] (US Und.) cross-examination of a prisoner.

In phrases

clean up one’s act (v.)

[1950s+] (orig. US) to modify or improve one’s behaviour.

disappearing act (n.)

see separate entry.

do a/an/the — act (v.)

[late 19C+] to perform in a given manner; usu. combined with a specific n. or proper name, which defines the ‘act’ in question.

do a brown (act) (v.)

[1980s+] (N.Z.) to act in a shy manner; to sulk.

do the — act (v.) (also play the — act)

[late 19C+] (US) to pretend to a particular (by context) style of behaviour, to assume (often deceitfully) specific characteristics; in comb. with n., adj. or proper name.

get one’s act together (v.)

[1970s+] (orig. US black) to calm down, to plan sensibly, to state a goal and aim for it.

have one’s act together (v.)

[1970s+] (orig. US black) to be in full control of a situation, whether emotional, social, sexual, financial etc.

pull a — act (v.) (also pull the — act)

[1930s+] to pretend to behave in a specific way, to perform a certain routine.

pull an act (v.)

[1930s] to put on a show with the intention of deceiving or defrauding someone.

put on an act (v.) (also put on the act)

[1930s+] to show off, to talk for display, to behave insincerely.

queer someone’s act (v.) [queer v. (6)]

[1910s+] (US) to interfere, to spoil someone’s plans.

SE in slang uses

In phrases

on the act

[late 19C] working as an actor.