An extremely powerful coffee, will make the drinker knurd, and likely a powerful contributor to the philosophies and mathematical advances found among the peoples of the continent of Klatch.
It is believed that Achmed The Mad was drinking Klatchian Coffee when he wrote the Necrotelicomnicon. One of several places to purchase it from in Ankh-Morpork is Fat Sally's, on Squeezebelly Alley.
Klatchian coffee is a very obvious reference to Turkish coffee, which itself is a strong sobering beverage, but probably nowhere near the same level.
It could also probably be related to the term coffee klatch, which refers to any social gathering at which coffee is served.