Merriam-Webster's Collegiate® Dictionary, 11th Edition
DULL
I. adjective
Etymology: Middle English dul; akin to Old English dol foolish, Old Irish dall blind
Date: 13th century
mentally slow ; stupid ,
2. slow in perception or sensibility ; insensible , lacking zest or vivacity ; listless , slow in action ; sluggish ,
4. lacking in force, intensity, or sharpness , not resonant or ringing , lacking sharpness of edge or point , lacking brilliance or luster , low in saturation and low in lightness, cloudy , tedious , uninteresting , dullness also dulness noun dully adverb Synonyms: see: dull see: stupid
II. verb
Date: 13th century
transitive verb to make dull , intransitive verb to become dull