A jockstrap is a piece of men’s underwear worn in sport to hold and protect the penis and testicles. It has a pouch at the front and elastic straps around the waist and between the legs. Players in baseball, cricket, martial arts and some other sports wear one, often with a hard cup slipped into the pouch for extra protection. The word is informal; shops may say athletic supporter. Though actually, the first part jock here is old slang for the penis, not the “jock” that means a sporty student — that later “jock” probably grew out of this garment. It is a practical thing, not a joke word, even if the class giggles.