noun
dimwit; simpleton; slow-witted person
A derogatory, colloquial noun for someone who is stupid or slow on the uptake. Often used in casual, sometimes insulting, speech.
あいつは脳たりんだから、何を言っても無駄だ。
That guy's a dimwit, so it's no use telling him anything.
noun which may take the genitive case particle 'no', pre-noun adjectival
Used as a pre-noun adjective (e.g., 脳たりんな奴) to describe someone or something as stupid or idiotic. Strongly derogatory.
そんな脳たりんな考えは捨てろ。
Get rid of that stupid idea.
Derived from 脳 (のう, 'brain') + 足りん (たりん), a colloquial contraction of 足りない (たりない, 'insufficient'). Literally 'brain-deficient', used as a derogatory term for a stupid person.