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]
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.