The way The "goal" Background Chinese texts Classical Post-classical Contemporary Zen in Japan Seon in Korea Thiền in Vietnam Western Zen Jakugo (着語, capping phrase), or agyo (下語) of a kōan is a proof of solution of the case riddle, but not the solution itself.
In Zen Buddhism, kōan is used both as a meditation device and as an expression of Enlightenment (悟り/覚り, satori) – a radical experiential insight into the nature of things and the self alike.
A capping phrase is supposedly an articulation of such enlightening experience, most of the time in verse.
According to Victor Sōgen Hori the use of jakugo dates to the Song dynasty (AD 960 - 1270) and was developed from classical Chinese "literary games".
If the monk continues into advanced stages of the Rinzai Zen kōan curriculum, he will receive further literary assignments: the writing of explanations in Japanese, called kakiwake 書き分け, and the composition of Chinese-style poetry, called nenrō 拈弄 ...