Trousers derived from the words trouses - drawers, trousses - trunk hose, and/or trousse – to cover, truss. They were looser than the tight pantaloons that were favored for daytime wear while pantaloons were more appropriate for evening attire. Trousers were worn over breeches when horseback riding to keep the more formal clothes clean.
Some trousers in the 1820s to mid-century had loops (called stirrup straps or Gaiter bottom trousers) at the bottom of the trousers to go under the shoes and hold the pants down. The earlier versions went under the stocking rather than the shoe.
A "stirrup" is a footrest attached to a saddle. The loops called "Stirrup straps" were a common part of cavalry uniforms. They were intended to prevent the pants from hiking up the leg while riding. Cavalry units were considered the elite troops of their time. Dress uniforms for officer ranks often included elements borrowed from those typical of cavalry units. Also known as gaiter bottoms, the finest were cut as one piece along with the rest of the trousers. Some were sewn on after the trousers had been made. The fashion lived on as military uniform for a little longer than the 1870s until it was adopted by ski bunnies in the 1960’s.

Cuffs or "turn-ups" date to the 1860s when members of the Windsor cricket club began rolling up their trousers to protect them from mud and water. Consequently tennis players copied the look by rolling up their flannel trousers before hitting the courts.
English Royalty was seen in town wearing turn-ups and even to the Ascot races! British gentlemen began imitating the style, but the initial response was not positive.
There was pandemonium in the House of Parliament in 1893 when Viscount Lewisham appeared wearing cuffs on his trousers. Society disapproved of turn-ups, claiming that they collected dirt that would be brought indoors and that men had to take care to turn them down before entering a respectable indoor location.
The advantage of knee breeches worn in the 18th Century was that the hem was high enough off the ground that they were not likely to be soiled, and the stockings worn with knee breeches were much easier to launder.
But by 1880 tailors were putting cuffs onto trousers, and by the early 20th Century, cuffs had become an accepted variation on trouser bottoms.

Fashion Fundamental

Formal dress trousers are never cuffed, because there could not possibly be any chance of mud at a formal occasion!

The FLY is the opening at the front of your pants to make it easier to get on and off. Before flies on pants, various types of flaps were used. A variation of the modern fly, with buttons, was accepted by the French military in 1842. The use of the term "Fly" as a front opening for trousers probably came from the name of the flap of a tent. (Middle English flien, from Old English fl ogan.)

In the 1860’s knee pants were popular for sports 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 this style on the golf course.

They continued popularity through the 1920s and 30s when they became known as Knickerbockers after a common last name of the New York Dutch who wore traditional knee pants.

Oscar Wilde tried to reintroduce breeches in 1890, but the gentlemen of the day were rational in their rejection. The state of mind held steady until 1925 when a hot summer was the excuse for Oxford Bags. The measurement of these loose pants at the leg bottom reached 40 inches!! Invented and embraced by English Oxford University students, Bags were inspired by the loose trousers that oarsmen slipped on over their shorts. The extreme fashion didn’t last long, but reappeared as the pants to wear with the Zoot Suit in 1938.

Although not as extreme, another attempt at wide bottoms came when Pierre Cardin popularized bell-bottoms in the 60’s as a reaction to the new narrow shoulder suits. Jeans were also effected and affected during that time.

Another word, which is interchangeable with pants and trousers, is slacks, which was coined by the Haggar Corporation in the 1940s as a promotion for their casual pants, to be worn during your "slack" time between work and sports. Ed Haggar and Morris Hite are credited with coming up with the word.

Short pants were also an English military invention to keep cool defending the far flung Empire. Bermuda shorts were worn down to the knee and are named after the British island.