鈍 - dull, slow, foolish

becoming dull; slowing down

thickheaded; obtuse; stupid

blunt weapon

thickheaded; insensitive; dull; thick-skinned

dull (e.g. a knife); blunt

to become blunt; to grow dull

blockhead; dunce; dimwit; dolt; slowcoach; slowpoke

The Power of Insensitivity (book by Junichi Watanabe)

to become dim-witted; to lose one's edge; to become insensitive

dull-witted; stupid; foolish

to anneal

obtuse angle