Nichtlachen-Keinwortz Syndrome

The medical condition which has deprived Mavolio Bent of any discernible sense of humour.

The Überwaldean appears to translate as "not laughing - no joke", although "wortz" is perhaps an old or unusual dialect. (See discussion.)

Annotation

The very free translation (in the spirit of Discworld "foreign languages") might be "Cannot Get The Joke syndrome".

Nichtlachen-Keinwortz Syndrome seems to be the Discworld equivalent to Asperger syndrome.