1. dull (e.g. a knife); blunt
鈍いナイフでは肉がうまく切れない。You can't cut meat well with a dull knife.
2. thickheaded; obtuse; stupid
ぼくは鈍いから、どうせわからないさ。I'm slow on the uptake, so I won't understand anyway.
3. dull (sound, color, etc.); dim (light)
なんだか音が鈍い。The sound seems dull somehow.
4. slow; sluggish; inert; lethargic
しかし、思ったより燃え方が鈍い。However, it burns more slowly than I thought.
5. insensitive; dull (e.g. reflexes); unperceptive; unfeeling
私はどうも人の気持ちに鈍いようでな。I seem to be rather insensitive to people's feelings.