A speedo is a style of swimwear for men and boys which is akin to briefs in terms of how it's shaped. It derives its name from the Speedo company, which first introduced that particular style of swimwear, although people generally refer to all brief-cut bathing suits as speedos, even if they aren't made by the Speedo brand. The fabric of speedos is comprised of lycra/spandex and nylon, making them quick-drying, smooth to touch, and form-fitting.
Speedos are quite a common swimwear choice for men and boys who live in Europe, South America, Australia, and Asia (especially Russia). Unfortunately, for some bizarre reason, speedos aren't well received in North America despite them being sexier in style than conventional swimming shorts.
Speedos have even managed to make their way into rules and laws of certain establishments and areas. There are many swimming pools in Europe (especially France), which require all males to wear speedos for sanitary purposes. The reason is because males may be in dirty areas outside the pool facility while wearing conventional swimming shorts and then track the filth into the pool; but it is assumed that with speedos, a pair of pants is worn over them until it's time for swimming, thus making speedos less exposed to the elements than regular shorts.
On the opposite end of the spectrum, the town of Cape May New Jersey USA had banned men from wearing speedos in public. In the 1960's, the South Jersey beach town made a law stating that any male over 12 years of age could not wear swim briefs. This ban was passed mainly because of homophobic claims that speedos were essentially worn by gay men for no other reason but to advertise their 'goods' to each other. The ban was finally lifted in April 2005, more than three decades later.
To sum up, speedos are the sexiest swim attire for men and the world needs more of them.
"Damn, look at that hot boy in speedos over there. He knows he's looking fine."