adverb
at the very beginning of one's speech; immediately upon opening one's mouth; first of all
Used when someone starts speaking and the first thing they say is notable, often surprising or direct. Common in narrative and reported speech.
彼は開口一番、辞職すると言った。
He announced his resignation the moment he opened his mouth.
開口一番、母は私の成績のことを聞いてきた。
The first thing my mother said was to ask about my grades.