The term has two meanings. First, it is the area immediately offstage where actors wait to make their entrances. They wait "in the wings," because the second meaning is the painted canvas flats in a 19th-century wing-and-drop setting. Later varieties of stage settings might still use wing flats painted black to mask offstage areas on either side of the set. Before the advent of strict union regulations, theatres were remarkably casual about allowing special guests to stand in the wings to watch the show. There are numerous tales of actors who could play excruciatingly poignant scenes on stage and then drop character instantly as they moved into the wings. An actor who did not know his lines would contrive to get through a performance by keeping his sides in the wings to consult between entrances on stage; this practice was known as "winging it."
The Historical Dictionary of the American Theater. James Fisher.