painful reproach; a needle in one's scalp
Idiomatic expression for a sharp, stinging criticism that hits a weak point or exposes a fault. The literal image is of a needle piercing the top of the head.
His remark was truly a painful reproach.
上司からの一言が頂門の一針となり、自分の甘さに気づいた。
A single word from my boss became a needle in my scalp, making me realize my own naivety.