Here are a few of my favorite entries, all beginning with the letter “K," from Word and Phrase Origins, a thoroughly delightful book published in 1997.
Katydid. John Bartram, America's first great botanist, first recorded katydid in 1751 as the name of the large green insect of the locust family. The word is imitative in origin, as the chattering noise the insect makes sound like "Katydid! Katydid!"
Khaki. The khaki cloth used for soldiers' warm-weather uniforms owes it name to its color. Khaki, twilled cloth originally made in India, means "dust-colored or dusty," deriving the Persian khak (earth).
Kith and kin. This expression, still heard today, dates back to the 14th century or earlier. It means acquaintances and relatives, kith being Middle English for friends and neighbors and kin (of course) meaning family.
Knapsack. Knapsacks were originally small sacks that German soldiers used to carry their rations. The word derives from the German knappen (to eat).
K-9 Corps. The army's K-9 Corps, organized during World War ll, was originally called D4D, an acronym for "Dogs for Defense." But the much cleverer K-9 Corps, a pun on "canine" quickly replaced the original term.
A note about the author: Word and Phrase Origins was written by Robert Hendrickson, the author of more than 25 other books, including American Literary Anecdotes, New York Tawk, and More Cunning than Man: A Social History of Rats and Men.