A euphemism for Macbeth, the play by William Shakespeare.
Theatre people have a superstition against mentioning the word Macbeth, or even quoting from the play, while within the bounds of a theatre. When there is need to refer to that play, actors will almost always use the alternative phrase, The Scottish Play or The Bard's play.
While there are many alternative explanations for the origins of this superstition, the most likely appears to be relatively unknown, even among theatre people.
Some say the spell uttered by the witches, "Round about the cauldron go; In the poisn'd entrails throw..." is a real spell and will inevitably bring doom upon those who are close by when the spell is cast. The hardcore rationalist in me wonders why it might be that the word Macbeth became taboo, when the alleged power-words are something quite different.
Others suggest that, as a study of the evils of unchecked ambition, there are dark forces surrounding the play and its famous speeches, and this bad karma gets translated into bad luck for performers and back-stage crew alike. Again, the rationalist in me wonders why, when mere mention of the name will bring ill-fortune, do actors vie for the opportunity to play the lead roles of the cynical murderer Macbeth, and his equally-ambitious wife, Lady Macbeth.
There are tales of stage daggers being replaced with real, pointy ones for the murder scene and actors dying on stage. Not so much bad luck as bad planning, I'd say.
Then there is the more believable suggestion that accidents really do happen, but that might be expected, as the stage is dark, and if a fog machine is being used, that can make the stage slippery. There are fires and weapons and other props which can lead to injury among people who are otherwise pre-occupied. rootbeer277 reminds me that The Straight Dope says it's a bit of a self-fulfilling prophecy. The play is performed so often, and actors are constantly on their guard in case something untoward happens, that, well, it does seem to happen. And even if it happens no more than statistics might predict, the actors always remember the time the lights went out during Macbeth, even if they would forget the same incident had it happened during The Merry Wives of Windsor, for example.
A more plausible explanation, taken both from Clement Freud on last week's Just a minute and a number of other sources, is far less romantic.
Historically, Macbeth has always been a popular play among audiences. There is lots of blood and gore in it. There are Kings and noblemen and valiant warriors and there are witches and magic and dark deeds. The characters are dreadfully flawed and give scope for actors to show off their skills at portraying dread, ambition, fear and other powerful emotions. These things have always been -- and remain -- popular with audiences.
When a company had a theatre booked, but their pre-planned production was not going well, the director would sometimes cancel the main performance and replace it with a run of a surefire winner: Macbeth. Similarly, when a theatrical company was in danger of going bust, they would choose to put on a performance of Macbeth, in order to generate enough money to pay off the debts.
Thus, the mere mention of the word, "Macbeth" when spoken out loud in a different theatrical context spread fear that the current play or performance would be a failure. Even worse, it came to suggest that the company might be headed for bankruptcy. A terrible -- and all-too-common -- fate for actors in the 18th and 19th centuries.
Nevertheless, theatrical superstitions have a habit of sticking. So let me spell it out. Amongst theatrical folk, do not, ever, mention the world Macbeth or quote from the play within the confines of a theatre and especially backstage. The only exception to this rule is during rehearsals. And then only when the line is scripted. Unless, of course, your name is Homer Simpson.
If you do break the taboo, the remedy is well-known: Leave the room, spin around three times whilst swearing, spit over your left shoulder and then knock on the door and await an invitation before returning to the room. Works every time!
Sources:
http://www.endicott-studio.com/rdrm/theatricalSuperstitions1.html
http://www.lyttonplayers.co.uk/Previous%20Productions/Macbeth/macbeth.htm http://www.straightdope.com/mailbag/mmacbeth.html | 
