MDMD "Chao-chou, dog"

[Terrance]

MU Zen Japanese (Chinese, wu), nothing, not, nothingness, un-, is not,
has not, not any; 
the wato of the famous koan "Chao-chou, dog," often called the koan mu
(Wu men kuan). 

Encyclopedia of Eastern Philosophy and Religion

Tarrant, John. 1997. The koan 'Mu'. Wu-Men Kuan, case #1. California
Diamond Sangha Newsletter. Summer 1998 [Available online from URL] 

http://www.ciolek.com/WWWVLPages/ZenPages/Koans-WMK.html#About