A surname from German.

plural of Dulle

Dim-witted, stupid, unintelligent

Having a dull, blunt wit or mind; unintelligent.

(simile) Alternative form of dull as dishwater

(simile) Boring; ordinary; exceedingly dull.

dull-witted; not clever

A dull, uninteresting person.

(there is) always something interesting happening; (there is) no risk of boredom

Too much inactivity makes one less mentally acute.