mid-15c., "to act upon or adjudicate" a legal case; 1590s in the theatrical sense, from Latin actus, past participle of agere (see act (n.)). To act up "be unruly" is from 1903. To act out "behave anti-socially" (1974) is from psychiatric sense of "expressing one's unconscious impulses or desires." Related: Acted; acting.[act etymology, act origin, 英语词源]