WordNet sense 1 (deficient in human sensibility; not mentally or morally sensitive):

insensitive

WordNet sense 2 (deaden (a sound or noise), especially by wrapping):

damp, dampen, muffle, mute, tone down

WordNet sense 3 (make numb or insensitive):

benumb, numb

WordNet sense 4 (not clear and resonant; sounding as if striking with or against something relatively soft):

thudding

WordNet sense 5 (not reverberant; lacking a tendency to reverberate):

nonresonant, unreverberant

WordNet sense 6 (being or made softer or less loud or clear):

muffled, muted, softened

WordNet sense 7 ((of sound) relatively low in volume):

soft

WordNet sense 8 (so lacking in interest as to cause mental weariness):

boring, ho-hum, irksome, slow, tedious, tiresome, wearisome, deadening

WordNet sense 9 (lacking brilliance or vitality):

lackluster, lusterless

WordNet sense 10 (tediously repetitious or lacking in variety):

humdrum, monotonous

WordNet sense 11 (lacking lightness or liveliness):

heavy, leaden

WordNet sense 12 (lacking in liveliness or charm or surprise):

dreary

WordNet sense 13 (lacking vitality or spirit; lifeless):

arid

WordNet sense 14 (used of a knife or other blade; not sharp):

blunt

WordNet sense 15 (become less interesting or attractive):

pall

WordNet sense 16 (darkened with overcast):

gray, grey

WordNet sense 17 (full of or covered with clouds):

cloudy

WordNet sense 18 (slow to learn or understand; lacking intellectual acuity):

dense, dim, dumb, obtuse

WordNet sense 19 (lacking or marked by lack of intellectual acuity):

stupid

WordNet sense 20 ((of color) not chromatically pure; diluted):

unsaturated

WordNet sense 21 (not brilliant or glaring):

subdued

WordNet sense 22 (not reflecting light; not glossy):

flat, matt, matte

WordNet sense 23 ((of business) not active or brisk):

sluggish

WordNet sense 24 (lacking activity; lying idle or unused):

inactive