6. A manifestation of intentional or unintentional insincerity; a pose: put on an act.
2. To express (unconscious impulses, for example) in an overt manner without awareness or understanding.
be in on the act
To be included in an activity.
clean up (one's) act Slang
To improve one's behavior or performance.
get into the act
To insert oneself into an ongoing activity, project, or situation.
get (one's) act together Slang
To get organized.
[Middle English, from Old French acte, from Latin āctus, a doing āctum, a thing done, both from agere, āct-, to drive, do.]