masc. proper name, Gaelic, literally "son of life." The first reference to bad luck associated with Shakespeare's "Macbeth," and to avoidance of naming it, is from 1896, alludes to an incident of 1885, and says the tradition goes back "so far as modern memory can recall." The original superstition seems to have pertained particularly to the witches' scenes, which were played up dramatically in 19c. productions, and especially to Matthew Locke's 17c. music to accompany the witches' song, which was regularly played through the 19th century.
It is strange how the effect of this music has exerted such a long surviving influence on members of the dramatic profession. It is still considered most unlucky to sing, hum, or whistle the witch airs in the theatre except in the ways of business. [Young-Stewart, "The Three Witches," in The Shakespearean, Sept. 15, 1896]
If you number an actor or actress among your friends, and desire to retain his or her friendship, there are three things you positively must not do, especially if the actor is of the old school. Do not whistle in the theatre, do not look over his shoulder into the glass while he is making up, and do not hum the witch's song from "Macbeth." ... [O]lder actors would almost prefer to lose their salary than go on in "Macbeth" on account of this song. They believe that it casts spells upon the members of the company. ["Some Odd Superstitions of the Stage," Theatre magazine, July 1909]