Translation guide
The English word "dull" covers several distinct meanings: lacking sharpness (blade, point), lacking brightness or luster (color, surface), lacking interest or excitement (boring), lacking intensity (pain, sound), and slow to understand (stupid). Each meaning has different natural Japanese expressions.
Describing a blade, knife, scissors, or point that is not sharp.
General adjective for dull, blunt, or slow. Used for blades, senses, movements, etc.
This kitchen knife is dull.
Literally "cutting quality is bad". A common, natural way to say a blade is dull.
This knife is dull.
Literally "cannot cut". Simple and direct way to say something is dull.
Those scissors are dull.
Describing a lack of luster, gloss, or brightness in colors, surfaces, hair, eyes, etc.
Past tense of くすむ (to become dull/dim). Describes colors or surfaces that lack brightness.
The wall color is dull.
Can also describe dull light, sound, or color, but less common for color than くすんだ.
Literally "has no luster". Used for hair, surfaces, etc.
My hair is dull.
Describing something that lacks excitement, interest, or stimulation.
Describing a pain, ache, sound, or weather that is not sharp, loud, or severe.
Used for dull pain (鈍い痛み), dull sound (鈍い音), etc.
A dull pain persists.
I heard a dull sound.
Describes dull, overcast weather or a dull, heavy atmosphere.
dull sky
Describing a person who is not intelligent or quick to understand.
鈍い (にぶい) is a general adjective for dullness in senses, movement, or mind. くすんだ is specifically for lack of brightness in colors or surfaces. For a dull blade, use 鈍い or 切れ味が悪い; for dull colors, use くすんだ.
Directly translating 'dull person' as 鈍い人 can sound unnatural. Use 頭が鈍い (slow-witted) or のろま (dim-witted) depending on context. For 'boring person', use 退屈な人 or つまらない人.
dull light
His story is dull.