noun, na-adjective
A mildly derogatory term for a foolish or slow-witted person; not extremely harsh but clearly insulting.
あいつは本当にうすばかだな。
That guy is a real dimwit.
うすばかなことを言うな。
Don't say such stupid things.
Compound of 薄 (usu, 'thin, weak') and 馬鹿 (baka, 'fool'), literally 'thin fool'. The exact origin is uncertain, but it likely developed as a colloquial insult.