1. Intellectually weak or obtuse; stupid; mentally very slow: Long hours working in the hot fields contributed to Nell's dullish behavior at the end of the day.
2. Lacking responsiveness or alertness; insensitive: Ray's dull response to the shattering news in the newspaper was unexpected.
3. Dispirited; depressed: Compared to other patients at the mental health clinic, Debora has a dull feeling.
4. Not brisk nor rapid; sluggish: Janine did not get a good night’s sleep last night and so she feels as if her brains are dull this morning.
5. Not having a sharp edge or point; blunt; such as, a dull knife: The knife was old and the blade was dull and not good for chopping vegetables.
6. Not intensely or keenly felt: Jacob has a dull headache today that simply won't go away.
7. Arousing no interest or curiosity; boring: It was a dull TV program.
8. A reference to a color; such as, a plain brown which is neither bright nor vivid: Helena's paintings were characterized by her use of dull colors with an occasional bright highlight.
9. Cloudy or overcast: Norbert looked out the window and saw a dull sky.
10. Not clear or resonant: Latonya heard a dull thud from the apartment above when someone up there dropped something.
11. Etymology: originally meant "slow-witted" and was borrowed from Middle Low German dul, and was a descendant of the prehistoric Germanic adjective dulaz.

It is recorded as having come from Middle English dulle, dull; from Old English dyl, a parallel form of Old English dol, "foolish, presumptuous", and related to Old English dwellan, "to lead astray".

1. To make less bright or to be less alert or keen; that is, to become or to cause something to become less clear, distinct, bright, or shiny: "The fog dulled the morning for a few hours until the sun came out."

"Special earplugs dulled the sound of the chain saw."

2. To make something; such as, a feeling less sharp, not as strong, or reduces severity: "The medication dulled her mind and hampered her thinking skills; however, it was also dulling her pain."

3. To become or to cause something; such as, a knife or blade to become less sharp: "This knife has been dulled because of so much usage."