adjective
duller; -est
not exciting or interesting :boring
having an edge or point that is not sharp
of a sound :not clear and loud
of pain :constant but not sharp or severe
not shiny
not sunny :having a lot of clouds
slightly grayish or dark :not bright
old-fashioned :stupid or slow in understanding something
not having a lot of business or financial activity
(as) dull as dishwater
(as) dull as ditchwater
verb
dulls; dulled; dulling
to become dull or to make (something) dull: such as
to become or cause (something) to become less clear, distinct, bright, or shiny [+ obj]
[+ obj] :to make (something, such as a feeling) less sharp, strong, or severe
to become or cause (something, such as a knife or blade) to become less sharp [+ obj]
US