Before the wonderful world of technology and advanced machinery, theatre companies employed theatrical fly systems made of rope and counter weights, to lift a production’s scenery or props into place.
The fly systems date back to the 1600’s, handled by backstage crewmen that would physically hoist items into the air. To do so safely, a combination of whistles were used as a cue to manually lift scenery into the air with ropes.
An actor who whistled backstage ran the risk of accidentally cueing stagehands to move, lift or drop scenery or props, which proved dangerous, and potentially life-threatening, for cast and crew that may be unaware of roaming items. To avoid creatives becoming the next 'theatre ghost', whistling was avoided altogether.
Decades later, it remains a tradition - with many deeming it an 'unlucky omen' for the performers and production.