This is the Japanese pronunciation of the Chinese character ‘wu’, which means ‘to lack’ or ‘there is not’. In ordinary usage it negates the presence of something. It is famous in Ch'an andzen circles as the ‘critical phrase’ (Chin., hua-t'ou) of the kōan ‘Chao-chou's dog’, the first case in the Mumonkan collection (see Gateless Gate). When asked whether or not a dog has Buddha-nature.Chao-chou replied, ‘It does not’ (wu/mu). Practitioners working with this kōan try to penetrate the meaning of this answer.