na-adjective
honest to a fault; foolishly honest; naively honest
Describes someone whose excessive honesty leads to disadvantage or social awkwardness. Often used critically or affectionately.
彼は馬鹿正直だから、秘密をすぐに話してしまう。
He's honest to a fault, so he ends up spilling secrets right away.
馬鹿正直
I answered everything with foolish honesty and failed the interview.