daffy (adj.)youdaoicibaDictYouDict
1884, perhaps from daft (adj.), or from obsolete daffe "a halfwit" (early 14c.; mid-13c. as a surname). Compare late 15c. daffish "dull-witted, spiritless." With -y (2). Related: Daffily; daffiness.
dull (adj.)youdaoicibaDictYouDict
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 (cognates: 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.
dunce (n.)youdaoicibaDictYouDict
"dullard," 1570s, from earlier Duns disciple "follower of John Duns Scotus" (c. 1265-1308), Scottish scholar of philosophy and theology supposed to have been born at Duns in Berwickshire. By 16c., humanist reaction against medieval theology singled him out as the type of the hairsplitting scholastic. It became a general term of reproach applied to more conservative philosophical opponents by 1520s, later extended to any dull-witted student.