About 1760 most men begin wearing breeches, a tight garment worn from the waist to the knee with stockings covering the rest of the leg, "Britches" was an informal word for breeches. Prior to this men were wearing various form if skirts and dresses.
Pantaloons (where we get the word pants) were made popular in 1812 by George Bryan "Beau" Brummell who wore his with a foot strap (like modern ski pants) to keep the pants tight and avoid creases. Brummell, buddy to the future Hind George IV, developed a dress code that anyone, not just royalty, could follow. He dispensed fashion tips and stressed cleanliness (a novel idea for the time).
Pantaloon first appeared as an English word in the 1600's and from the Italian comedy character Pantaleone who wore the first loose "clowns pants". Eventually the characters name came to mean the pants he wore. In England pants still refers only to underwear.
The French revolution of 1789 was also a revolt against breeches as being too upper class. The country peasant trouser look was in.
Trousers probably derived from the words trousers-- drawers, trousses--trunk hose, and/or trousse--to cover, truss. They were looser than the tight pantaloon were favored for daytime wear while pantaloons were more evening attire. Trousers were over breeches when horseback riding to keep the more formal clothes clean.
Sailors had been wearing the looser fit work trousers since the 1580s since they allowed them to roll up the legs for wading ashore or climbing rigging.
In 1846 Sir Harry Lumsden, commanding as English troop in Punjab, India traded in his bright white trousers for pajama bottoms to find relief from the heat. To disguise them he colored them to blend with the local terrain using mazari, a native plant. Thus the birth of Khaki, the Hindu word for "dust". As a by product, Lumsden discovered that the new Khaki pants were more suitable in battle than the white pants, and red tunic. Blending in was good. Khaki is a color, but is now synonymous with a military twill pant.
Khaki went from India to the Kaffir War in South Africa in 1851, and then after the Sudan Wars and Afghan Campaign of 1878 it was adopted in 1884 as official uniform. The same year khaki-order dye was adopted by other armies including America for the Spanish-America War in 1898.
Although not all armies were as willing to give up their brightly colored uniforms:
"Les pantaloons rouge, ils sont la France!" - Members of the French Army
Chinos were military issue pants which were originally of Chinese made in China. The British Khakis found their way into China they were duplicated and sold to American soldiers in the Philippines for uniforms. Chinos don't have to be twill, but are often a firm weave of cotton. Chinos can be khaki color. The military style had no pleats and was tapered at the leg bottom to conserve fabric. When soldiers returned to civilian life from WWII they continued to wear their military chinos especially to college.
Reportedly, Bill's khakis is one of the best pants on the market. And L. L. Bean stocks jeans to chinos.
Can we talk about ladies bloomers? They were pants invented by Elizabeth Smith Miller and consisted of a short skirt with baggy trousers gathered at the ankles. This masculine article of clothing appealed to Amelia Jenks Bloomer of Homer, New York who adopted and popularized the style as kind of rebellion about 1850. They were embraced by the first women's liberation troops and sports-minded ladies who rode the bicycle craze of the 1890's.
In the 1860's knee pants were popular for sorts such as hunting and golf. They took the form of loose breeches such as "plus fours" which came four inches below the knee. We still see on the golf course. They continued popularity through the 1920s and 30s when they became known as knickerbockers after a common last of the New York Dutch who wore traditional knee pants.
Short pants were also an English military invention to keep defending the far flung Empire. Bermuda shorts were won down to the knee and named after the British island.