noun
unsolicited explainer; know-it-all who lectures others
Colloquial, mildly derogatory term for someone who eagerly explains or teaches things without being asked, often seen as annoying.
彼はすぐに教え魔になるから、質問しにくい。
He quickly turns into an unsolicited explainer, so it's hard to ask him questions.
飲み会で隣の席の人が教え魔で、ずっと話を聞かされた。
At the drinking party, the person next to me was a know-it-all lecturer and I had to listen to him talk the whole time.
Compound of 教え (oshie, 'teaching') + 魔 (ma, 'demon' or 'fiend'), suggesting someone possessed by a demon-like urge to teach. The exact origin is uncertain, but it follows a pattern of colloquial -魔 compounds describing obsessive behavior.