noun
northeastern unlucky direction; demon's gate
In traditional onmyōdō cosmology, the northeast direction is considered unlucky and is called the demon's gate. Figuratively, it can refer to a person or thing that brings bad luck or should be avoided.
家を建てるときは鬼門を避ける人が多い。
Many people avoid the northeastern direction when building a house.
彼にとってあの上司は鬼門だ。
That boss is a person to be avoided for him.
noun
Used for a personal weakness, a subject one is bad at, or a situation one finds difficult to handle. Often appears in phrases like 鬼門だ or 鬼門になる.
数学は昔から鬼門だ。
Math has always been my weak point.
彼にとって人前で話すのが鬼門らしい。
Speaking in public seems to be his Achilles' heel.
From the on'yomi reading of 鬼 (demon) and 門 (gate), originally referring to the northeast direction in onmyōdō, which was considered the direction from which demons enter. The figurative meanings developed from this unlucky association.