“The Scottish Play” is a euphemism for Shakespeare's play Macbeth, used by many actors when they want to refer to the play without saying the name, out of respect for the superstitions which surround the play. According to legend, saying the name of the play or quoting from it inside a theater can bring down bad luck on the theater and the actors. The exception to this rule is, of course, when an actual performance of Macbeth is occurring, in which case obviously actors must quote from the play.
The superstitions surrounding the Scottish Play are well-established in the theater community, but their origins are a bit unclear. Supposedly, Macbeth has been plagued with accidents, deaths, and runs of bad luck for theaters, although empirical analysis does not seem to suggest that the Scottish Play is any more dangerous to perform than any other play. Some people have suggested that the witches in the play are casting real spells, or that real witches were offended by the content of the play, and they put a curse on it. More practical historians have suggested that the large number of fight scenes in the play does put people at a risk of accidents.
Macbeth has also historically been used by struggling theater companies to draw in audiences in an attempt to make a season profitable. As a result, a high proportion of theaters historically have failed after a performance of the Scottish Play, but not because of the play. Rather, the decision to perform the play was a symptom of the larger problem.
Actors in general are extremely superstitious, and the beliefs about the Scottish Play are only a small part of the complex fabric of superstitions which can be found in theaters large and small. If someone does break the Macbeth rule, he or she is usually obliged to perform some sort of ritual to ward off the bad luck, such as leaving the room and knocking for admittance, turning around three times, spitting over the right shoulder, or quoting a line from another Shakespeare play..
Older actors often tell new and young actors horror stories about the Scottish Play, with many actors regarding this as a rite of passage involved in the initiation into the theater community. This euphemism references the Scottish setting of the play, as do other alternate names like “Macker's,” or “The MacScottish Play.” Other actors may refer to it simply as “The Bard's Play,” or “that one play.” The lead character, Macbeth, is often called “Mr. M,” to avoid saying the M-word, and his wife is known as “Lady M.”