Knickerbockers are men's or boys' breeches or baggy-kneed trousers particularly popular in the early twentieth century USA. Golfers' plus twos and plus fours were breeches of this type. Before World War II, skiers often wore knickerbockers too, usually ankle-length.
Until after World War I, in many anglophone countries, boys customarily wore short pants in summer and knickerbockers or "knickers" (or "knee pants") in winter. At the onset of puberty, they graduated to long trousers. In that era, the transition to "long pants" was a major rite of passage. See, for example, the classic song Blues in the Night by Johnny Mercer: "My mammy done told me, when I was in knee-pants, my mammy done told me, son...".
Baseball players wear a stylized form of knickerbockers, although the pants have become less baggy in recent decades and some modern ballplayers opt to pull the trousers close to the ankles. The white trousers worn by American football officials are knickerbockers, and while they have become less baggy, they are still worn ending shortly below the knee. In recent years, the NFL has equipped its officials with long trousers rather than knickers in cold weather.
The name "Knickerbocker" first acquired meaning with Washington Irving's History of New York, which featured the fictional author Diedrich Knickerbocker, an old-fashioned Dutch New Yorker in Irving's satire of chatty and officious local history. In fact, Washington Irving had a real friend named Herman Knickerbocker (1779–1855), whose name he borrowed. Herman Knickerbocker, in turn, was of the upstate Knickerbocker clan, which descended from a single immigrant ancestor, Harmen Jansen van Wijhe Knickerbocker. Jansen van Wijhe invented the name upon arriving in New Amsterdam and signed a document with a variant of it in 1682. After Irving's History, by 1831, "Knickerbocker" had become a local bye-word for an imagined old Dutch-descended New York aristocracy, their old-fashioned ways, their long-stemmed pipes, and knee-breeches long after the fashion had turned to trousers. (Such cultural heritage sprang almost entirely from Irving's imagination and became a well-known example of an invented tradition.) "Knickerbocker" became a byword for a New York patrician, comparable to a "Boston Brahmin".
Thus the "New York Knickerbockers" were an amateur social and athletic club organized by Alexander Cartwright on Manhattan's (Lower) East Side in 1842, largely to play "base ball" according to written rules, the first organized team in baseball history; on June 19, 1846 the New York Knickerbockers played the first game of "base ball" organized under those rules, in Hoboken, New Jersey, and were trounced 23 – 1. The Knickerbocker name stayed with the team even after it moved its base of operations to Elysian Fields at Hoboken, N.J. in 1846. The baseball link may have prompted Casey Stengel to joyously exclaim, "It's great to be back as the manager of the Knickerbockers!" when he was named pilot of the newborn New York Mets in 1961.
Hence also the locally-brewed "Knickerbocker Beer" brewed by Jacob Ruppert, the first sponsors of the tv show tonight!; hence the gossip columnist "Cholly Knickerbocker", the pen name of Igor Cassini; hence the extremely high-toned Knickerbocker Club (still in a neo-Georgian mansion on Fifth Avenue at 62nd Street, which was founded in 1871 when some members of the Union Club became concerned that admission policies weren't strict enough); and hence the New York Knicks, whose corporate name is the "New York Knickerbockers."
The Knickerbocker name was an integral part of the New York scene when the Basketball Association of America granted a charter franchise to the city in the summer of 1946. As can best be determined, the final decision to call the team the "Knickerbockers" was made by the club's founder, Ned Irish. The team is now generally referred to as the Knicks.