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Consider the following scale:

• Being drunk is to be intoxicated by alcohol to such an extent as to be unable to perceive the world clearly through the senses.
• Being sober is to be able to perceive the world clearly through the senses, yet humans are quite capable of giving themselves illusions and little stories to make life more bearable.
• Being knurd is to be (un)intoxicated with Klatchian Coffee to such an extent that all such comfort stories are stripped away from the mind.
  This makes you see the world in a way 'nobody ever should', in all its harsh reality.

People often find being knurd as painful, and hurry to get another drink (alcoholic this time). There are also some unfortunate individuals (for example, Sir Samuel Vimes of the Ankh-Morpork City Watch) whose bodies don't make enough natural alcohol, and are slightly knurd all the time until they have had a drink. Mis-dosing in an attempt to treat natural knurd-ness can lead to pathological alcoholism, or merely pathological violence as in the case of the Luggage, when given a handy chimera or basilisk to take its anger out on, following a prolonged session of getting knurd on coffee with orakh chasers. (the Klatchian method favoured in the city of al-Khali)

Knurd is, of course, "drunk" spelled backwards.