dull

[1]	muffle , mute , damp , dampen , tone down
[2]	numb , benumb , blunt
[3]	blunt
[4]	pall
[5]	allay , assuage , relieve , mitigate , lessen , reduce
[6]	dim , tarnish , obscure , bedim , blur , cloud , becloud
[7]	stupefy , narcotize , numb , benumb , desensitize , deaden , blunt , obtund

adj.

[1]	stupid , slow-witted , dense , stolid , bovine , cloddish , clod-like , backward , obtuse , doltish , crass , dumb , (Colloq) thick , dim , dim-witted , (Brit) dim as a Toc H lamp
[2]	insensitive , numb , insensible , imperceptive/impercipient , unresponsive , indifferent , unfeeling , unsympathetic , callous , hardened , hard , inured , obtundent
[3]	lifeless , indifferent , unresponsive , sluggish , slow , listless , inactive , torpid
[4]	boring , tiresome , tedious , monotonous , uninspired , uninspiring , unoriginal , uninteresting , humdrum
[5]	dismal , dreary , depressing , sombre , grey , dark , murky , gloomy , cloudy , clouded , overcast , sunless
[6]	blunted , blunt , obtuse
[7]	hazy , blurry , opaque , drab
[8]	muffled , numbing , deadened , muted , indistinct
[9]	muffled , muted , softened
[10]	boring , deadening , ho-hum , irksome , slow , tedious , tiresome , wearisome
[11]	dense , dim , dumb , obtuse , slow
[12]	slow , sluggish
[13]	thudding
[14]	leaden