Etymology
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dull (adj.)

c. 1200, "stupid, slow of understanding, not quick in perception;" also, of points or edges, "blunt, not sharp;" apparently from Old English dol "dull-witted, foolish," or an unrecorded parallel word, or from Middle Low German dul "foolish, reckless," both from Proto-Germanic *dulaz (source also of Old Frisian dol "reckless," Middle Dutch dol, dul "stupid, foolish, crazy," Old Saxon dol, Old High German tol "foolish, dull," German toll "mad, wild," Gothic dwals "foolish").

This sometimes is conjectured to be from PIE *dhul-, from root *dheu- (1) "dust, vapor, smoke," which also produced words for "defective perception or wits, turbidity of the mind" (compare Greek tholos "mud dirt," Old Irish dall "blind").

Dull. Ineffective for the purpose aimed at, wanting in life. A dull edge is one that will not cut ; a dull understanding, does not readily apprehend ; a dull day is wanting in light, the element which constitutes its life ; dull of sight or of hearing is ineffective in respect of those faculties. [Wedgwood]

From late 12c. as a surname. Rare before mid-14c. Of color "not bright or clear," from early 15c.; of pain or other sensations, "not sharp or intense," from 1725. Sense of "not pleasing or enlivening, uninteresting, tedious" is from c. 1400. Related: Dullness.

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

c. 1200, "to lessen the vigor, activity, or sensitiveness of" (transitive), from dull (adj.). Of pointed or edged things, "make less sharp, render blunt," from late 14c. Of colors, glass, etc., "remove the brightness or clearness of," late 14c. Intransitive sense of "lose vigor, intensity, or keenness" is from late 14c. Related: Dulled; dulling.

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dully (adv.)

"in a dull manner, stupidly," early 15c., from dull (adj.) + -ly (2).

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dullard (n.)

"stupid person, dunce, simpleton," mid-15c. (but early 13c. as a surname), from dull (adj.) + -ard.

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dullsville 

also Dullsville, "town where nothing happens," 1960, slang, from dull (adj.) + place-name ending -ville.

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dolt (n.)

"dull, stupid fellow," 1540s, perhaps a variant of dold "dull, foolish" (mid-15c.), influenced by dulte, dolte, past-participle forms of Middle English dullen "to dull; make or become dazed or stupid" (see dull (v.)). Related: Doltish "foolish, stupid" (1540s); doltishly; doltishness.

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stupid (adj.)

1540s, "mentally slow, lacking ordinary activity of mind, dull, inane," from French stupide (16c.) and directly from Latin stupidus "amazed, confounded; dull, foolish," literally "struck senseless," from stupere "be stunned, amazed, confounded," from PIE *stupe- "hit," from root *(s)teu- (1) "to push, stick, knock, beat" (see steep (adj.)). Related: Stupidly; stupidness.

Native words for this idea include negative compounds with words for "wise" (Old English unwis, unsnotor, ungleaw), also dol (see dull (adj.)), and dysig (see dizzy (adj.)). Stupid retained its association with stupor and its overtones of "stunned by surprise, grief, etc." into mid-18c. The difference between stupid and the less opprobrious foolish roughly parallels that of German töricht vs. dumm but does not exist in most European languages.

Men are born ignorant, not stupid; they are made stupid by education. [Bertrand Russell, paraphrasing Helvétius]
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doldrums (n.)

by 1803, "low spirits, the blues, the dumps," colloquial, probably from dulled, past participle of dull (v.) in the sense of "make (someone) slow-witted," with ending perhaps patterned on tantrum.

DEAR girl, from noise and London city,
I'm here among the blithe and witty;
Where young and old, from ev'ry clime,
Like adepts, learn to murder Time!
If you've the doldrums or ennui,
Forsake the town and come to me.
[from "A Marine Picture" in The Spirit of the Public Journals for 1802, London, 1803]

Transferred sense, in reference to sailing ships, "in a becalmed condition, unable to make headway" is by 1824. This was extended in nautical use to parts of the sea near the equator that abound in calms, squalls, and light, baffling winds (1848) and the weather characteristic of these parts. "Apparently due to a misunderstanding of the phrase 'in the doldrums', the state being taken as a locality" [OED].

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humdrum (adj.)

"routine, monotonous, dull, commonplace," 1550s, probably a reduplication of hum. As a noun, "monotony, tediousness," from 1727; earlier it meant "dull person" (1590s).

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hebetate (v.)

"make dull," 1570s, from Latin hebetatus, past participle of hebetare, from hebes "dull, blunt" (see hebetude). Related: Hebetated; hebetating; hebetation.

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