Major is a Latin word that means "greater" as compared to minor that means "less." As a military rank it started out in the Sixteenth Century or earlier as Sergeant Major, who was the "greater" of the Sergeants. We could also think of the Sergeant Major as the "big" or "top" Sergeant but in those days he was an officer, the second or third in command of a regiment or similar unit. The French started forming regiments in the Seventeenth Century by copying the Spanish technique of combining several companies into a column led by a Colonel. I will discuss the Colonel later. Sometimes the Captains of the companies making up the regiment would choose one of themselves as Colonel, another as Lieutenant Colonel and a third as Sergeant Major. Each would still be Captain of his own company. In practice the Colonel was often absent looking after his interests at court or playing politics for his own and his regiment's benefit leaving the Lieutenant Colonel as the effective commander of the regiment, aided by the Sergeant Major who was senior to the other Captains. An important part of the Sergeant Major's job was forming the companies into a regimental unit and keeping them in proper formation in a battle or on the march. A loud, commanding voice was the key to that task and one of the major qualifications for the post. A loud voice is still needed for the job.
As the regimental system became permanent during the Seventeenth and Eighteenth Centuries the Sergeant portion of the title gave way leaving just Major as the regiment's staff officer. Perhaps the other Captains objected to having a "big" Sergeant above them and other Sergeants below them. The title of Sergeant Major remained but as the top Sergeant among the common soldiers as he or she is today, although any good officer will admit that an effective Sergeant Major is still third in command of his regiment or other unit.
Majors in our Army started wearing oak leaves as rank insignia on their shoulder straps about 1832. Why the Army chose oak leaves remains a mystery. Navy and some Army officers had been wearing gold braid featuring oak leaves and acorns on their uniforms for several years. Generals, Admirals and some other senior officers still wear braid on their caps. One story has it that the Navy chose oak leaf braid as a tribute to the oak lumber used to build its ships. While that is a good story it ignores the fact that some British and French officers also wore braid with oak leaves and still do today. The British might have gotten the idea from the Germans who wore oak leaves in their headgear after a battle. That practice seems to go back a long time, perhaps to pagan warriors wearing the leaves as a tribute to whatever gods they worshipped. When the Elector of Hanover became King George I of Great Britain in 1714, his German followers might have introduced the oak leaf to the British military. Another story traces the British use of oak leaves to King Charles II who escaped from his enemies in 1650 by hiding in an oak tree. Anyway, back to the Major and his oak leaves. In 1832 the color of the leaves had to be opposite the color of the shoulder strap borders so Infantry Majors wore gold leaves while other Majors wore silver. After 1851 all Majors wore gold oak leaves. They did not have oak leaves on their epaulettes because the size of the fringes on their epaulettes and other features of their uniforms identified them as Majors.
From: Why is the Colonel Called "Kernal"? The Origin of the Ranks and Rank Insignia Now Used by the United States Armed Forces
Information borrowed Naval Historical Center
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