dull

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Arousing little interest; lacking liveliness; boring.
• adjective Not brisk or rapid; sluggish.
• adjective Not having a sharp edge or point; blunt.
• adjective Not intensely or keenly felt.
• adjective Not bright, vivid, or shiny.
• adjective Cloudy or overcast.
• adjective Not clear or resonant.
• adjective Intellectually weak or obtuse; stupid.
• adjective Lacking responsiveness or alertness; insensitive.
• adjective Dispirited; depressed.
• transitive & intransitive verb To make or become dull.

from The Century Dictionary.

• Stupid; foolish; doltish; blockish; slow of understanding: as, a lad of dull intellect.
• Heavy; sluggish; drowsy; inanimate; slow in thought, expression, or action: as, a surfeit leaves one dull; a dull thinker; a dull sermon; a dull stream; trade is dull.
• Wanting sensibility or keenness; not quick in perception: as, dull of hearing; dull of seeing.
• Sad; melancholy; depressed; dismal.
• Not pleasing or enlivening; not exhilarating; causing dullness or ennui; depressing; cheerless: as, dull weather; a dull prospect.
• Gross; inanimate; insensible.
• Not bright or clear; not vivid; dim; obscure: as, a dull fire or light; a dull red color; the mirror gives a dull reflection.
• Not sharp or acute; obtuse; blunt: as, a dull sword; a dull needle.
• Not keenly felt; not intense: as, a dull pain.
• noun A noose of string or wire used to snare fish; usually, a noose of bright copper wire attached by a short string to a stout pole.
• To fish with a dull: as, to dull for trout.
• To make dull, stupid, heavy, insensible, etc.; lessen the vigor, activity, or sensitiveness of; render inanimate; damp: as, to dull the wits; to dull the senses.
• To render dim; sully; tarnish or cloud: as, the breath dulls a mirror.
• To make less sharp or acute; render blunt or obtuse: as, to dull a knife or a needle.
• To make less keenly felt; moderate the intensity of: as, to dull pain.
• To become dull or blunt; become stupid.
• To become calm; moderate: as, the wind dulled, or dulled down, about twelve o'clock.
• To become deadened in color; lose brightness.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To become dull or stupid.
• adjective Slow of understanding; wanting readiness of apprehension; stupid; doltish; blockish.
• adjective Slow in action; sluggish; unready; awkward.
• adjective Insensible; unfeeling.
• adjective Not keen in edge or point; lacking sharpness; blunt.
• adjective Not bright or clear to the eye; wanting in liveliness of color or luster; not vivid; obscure; dim
• adjective Heavy; gross; cloggy; insensible; spiritless; lifeless; inert.
• adjective Furnishing little delight, spirit, or variety; uninteresting; tedious; cheerless; gloomy; melancholy; depressing; ; hence, cloudy; overcast.
• transitive verb To deprive of sharpness of edge or point.
• transitive verb To make dull, stupid, or sluggish; to stupefy, as the senses, the feelings, the perceptions, and the like.
• transitive verb To render dim or obscure; to sully; to tarnish.
• transitive verb To deprive of liveliness or activity; to render heavy; to make inert; to depress; to weary; to sadden.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Lacking the ability to cut easily; not sharp.
• adjective Boring; not exciting or interesting.
• adjective Not shiny; having a matte finish or no particular luster.
• adjective Not bright or intelligent; stupid; slow of understanding.
• verb transitive To render dull; to remove or blunt an edge or something that was sharp.
• verb transitive To soften, moderate or blunt.
• verb intransitive To lose a sharp edge; to become dull.