Many theories exist, but this is the most popular one.
This superstition comes from the fact that many early cultures considered triangles to be sacred symbols of life. For Christians, a triangle represents the Holy Trinity. Since a ladder up against a wall forms a triangle, walking beneath it would be disrupting sanctity and risk holy wrath.
Early Christians considered a ladder resting against a wall to represent the crucifixion, and therefore, evil. For this reason, convicted criminals were forced to walk under the gallows ladder, the entranceway to eternal damnation and darkness.
- HowStuffWorks.com – Why is walking under a ladder supposed to be unlucky?
- TimelessMyths.co.uk – Walking Under a Ladder