noun
stupid person; slow-witted person; dimwit
Colloquial and mildly derogatory term for someone who is slow to understand or foolish.
あいつはうすらとんかちだから、何度言ってもわからない。
That guy is a dimwit, so no matter how many times you tell him, he doesn't get it.
そんな簡単なこともできないなんて、うすらとんかちだな。
You can't even do something that simple? What a slow-witted person.
とんかち alone can mean 'hammer' or, colloquially, 'blockhead'. うすらとんかち is a more emphatic and specific insult for a stupid person.
ばか is a more common and versatile insult for 'fool' or 'idiot'. うすらとんかち is more colloquial and less harsh, often implying slowness rather than outright stupidity.
The exact derivation is uncertain. It likely combines うすら (a prefix meaning 'slightly' or 'thin') with とんかち (a colloquial term for a blockhead or hammer), yielding a sense of 'a bit of a dimwit'.