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dull

adjective, dull·er, dull·est.

not sharp; blunt: a dull knife.
causing boredom; tedious; uninteresting: a dull sermon.
not lively or spirited; listless.
not bright, intense, or clear; dim: a dull day; a dull sound.
having very little depth of color; lacking in richness or intensity of color.
slow in motion or action; not brisk; sluggish: a dull day in the stock market.
mentally slow; lacking brightness of mind; somewhat stupid; obtuse.
lacking keenness of perception in the senses or feelings; insensible; unfeeling.
not intense or acute: a dull pain.

verb (used with or without object)

to make or become dull.

adjective

slow to think or understand; stupid
lacking in interest
lacking in perception or the ability to respond; insensitive
lacking sharpness; blunt
not acute, intense, or piercing
(of weather) not bright or clear; cloudy
not active, busy, or brisk
lacking in spirit or animation; listless
(of colour) lacking brilliance or brightness; sombre
not loud or clear; muffled
med (of sound elicited by percussion, esp of the chest) not resonant

verb

to make or become dull

adj.

c.1200, “stupid;” early 13c., “blunt, not sharp;” rare before mid-14c., apparently from Old English dol “dull-witted, foolish,” or an unrecorded parallel word, or from Middle Low German dul “slow-witted,” both from Proto-Germanic *dulaz (cf. Old Frisian and Old Saxon dol “foolish,” Old High German tol, German toll “mad, wild,” Gothic dwals “foolish”), from PIE *dheu- (1) “dust, vapor, smoke” (and related notions of “defective perception or wits”). Of color from early 15c.; of pain or other sensations from 1725. Sense of “boring” first recorded 1580s.

dull. (8) Not exhilarating; not delightful; as to make dictionaries is dull work. [Johnson]

Dullsville, slang for “town where nothing happens,” attested from 1960.

c.1200, “to grow weary, tire;” of pointed or edged things from c.1400; of the senses from 1550s; from dull (adj.). Related: Dulled; dulling.

adj.

Lacking responsiveness or alertness; insensitive.
Not intensely or keenly felt, as in pain.

In addition to the idiom beginning with dull

dull as dishwater

also see:

never a dull moment