(The) of an opera or theatre. A room provided for ladies where they can wait till their carriages are called. Called crush because the room is not only crowded, but all crush towards the door, hoping each call will be that of their own carriage. “Mrs. X.'s carriage stops the way,” “Lord X.'s carriage,” etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894