na-adjective, noun
stupid; dull-witted; thickheaded
Describes a person or their thinking as slow, dim, or lacking sharpness. Often used in casual, mildly derogatory speech.
あいつはぼんくらだから、説明しても無駄だ。
He's so thickheaded that explaining it to him is pointless.
ぼんくらな部下に手
I'm having trouble with my dim-witted subordinate.
noun
Refers to a person who is habitually stupid or slow on the uptake. Used as a mild insult.
そんな簡単なこともわからないなんて、お前はぼんくらだな。
You don't even understand something that simple? You're such a dimwit.
The origin is uncertain. It may be a compound of 盆 (ぼん, tray or Bon festival) and 暗 (くら, dark), but the connection to the meaning is unclear. The word is likely a colloquialism from the Edo period.