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 generally find being knurd excruciating, as their comfortable illusions are stripped away and all of life's terrors are exposed. (Is this what the Hiver feels like all the time?) When accidentally knurd, people hurry to get alcohol into them to restore the balance. In fact, they generally go too far, getting very drunk in order to a) make certain they aren't knurd, and b) get so comfortably illusory that the mind can't recall those terrible realities.
There are some unfortunate individuals (for example, Sir Samuel Vimes of the Ankh-Morpork City Watch) whose body does not make enough natural alcohol, and is consequently slightly knurd all the time until he has had a drink. Since he's promised Lady Sybil he'll stay off the sauce, this lack of comforting illusion is probably the reason for his core cynicism: whereas most of us nurture the hope that the best and happiest outcome is always an option, he knows things aren't going to be any good and all he can hope for is to make them the least worst. Another Likely Example is that of Lord Vetinari the current Patrician. This may explain his inability to get drunk, and the fact that even Vimes calls him a quote 'cynical bastard'.
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.