What is the origin of the term 'knickers'?
Knickers have quite a twisted word history. The first records of knickers referring to clothing come from the late 1800s. It is a shortened form of knickerbockers, a type of baggy breeches fastened at the knee. Knickerbockers got their name from Diedrich Knickerbocker, the fictional author of Washington Irving’s A History of New York (1809), in part of which he chronicles the history of Dutch settlers in America. The word Knickerbocker became a nickname for the descendants of those settlers and for the type of pants they traditionally wore. (Eventually, it became a nickname for anyone from New York and, much later, the name of a New York basketball team).
Knickers is a short way of referring to the pants called knickerbockers. This use was once common in the U.S. But in the U.K., knickers are always underwear. Sometimes, the word specifically refers to a kind of loose-fitting underwear similar to bloomers (knickerbocker-like pants once worn by women in the U.S. as an alternative to dresses). More informally, it can also refer to the kind of short-legged underwear often called boyshorts. Knickers can also be used in a general way to mean “panties.”
Knickers appear in a few British idioms, including most commonly getting one’s knickers in a twist (or bunch or knot), which means “to get overly upset” and is typically used in a rude command not to do that. (The U.S. equivalent is to get one’s panties in a twist/bunch/knot, reflecting the fact that panties are a close synonym of knickers.)