The latest entry at The Discouraging Word, “The koan of the meshuggeneh,” has this to say about the etymology of koan: “Koan comes straight from the Japanese, from ko, public, and an, variously defined by our usual dictionary sources as “matter, material for thought” (OED, AH) and “proposition” (M-W).” It bothered me that the second definition was so vague, and even more that the word was only traced back to Japanese when it was clearly a Sino-Japanese loan word—I expect dictionaries to be more precise these days. So I did a little research and discovered that the original Chinese word, gongan (kung-an for you unreconstructed Wade-Gilesians), meant ‘legal case’; it’s composed of gong ‘public’ and an ‘(legal) case, records’ (the links go to the characters, with translations and renditions in Cantonese, Hakka, Minnan, Wu, and Sino-korean as well; I take this opportunity to bow reverently in the direction of the online Chinese character dictionary, one of the best language resources on the net).
A more detailed description comes from this site:
In ancient China, the koan (Chinese: gongan) was an official document that handed down an important judgment, a final determination of truth and falsehood. Adapting and subverting this notion, Zen (Chinese: Chan) Masters to this day make use of all sorts of stories, problems and situations, the more shocking the better, in order to cultivate their students’ awareness. The method usually consists of a question and an enigmatic answer. It is believed that such answers arise from the mysterious, irrational or paradoxical nature of truth. Only an apparently illogical answer can reveal it.
This may be old hat to any Zen masters among you, but it was new to me. And if anyone knows how and when the semantic shift occurred, please share.