Roman Soldiers Were Taught to Step Off With the Right Foot
The Latin word for left is sinister, the source of the English word for something evil.

TED-Ed · A day in the life of a Roman soldier - Robert GarlandRoman soldiers were taught to start marching with the right foot, because the left was considered unlucky.
Centurions, the officers who commanded groups of 80 to 100 men, marched alongside their legionaries shouting the cadence: "Dex, sin, dex, sin." The words simply mean "Right, left, right, left."
In Latin, the word for left is sinister, the origin of the English word for something threatening or evil. The chant was a marching order that also followed the superstition, keeping the unlucky foot from leading the way.
Questions people ask
How far did Roman soldiers march in a day?
A full march was about 30,000 Roman paces, or nearly 36 kilometers, covered in a single day. Soldiers did it carrying armor and weapons weighing more than 20 kilograms, plus a backpack filled with tools and rations.
How was a Roman legion organized?
Each legion held close to 5,000 men. It was divided into cohorts of about 500, which were further split into centuries of 80 to 100 men, each led by a centurion.
Did Romans avoid the left in other parts of life too?
Yes. Romans treated many left side occurrences as bad omens, from entering a room to seeing birds fly on the left. The bias survived in language, giving English both sinister, from the Latin for left, and dexterous, from the Latin for right and skillful.