The onset of something is the beginning of it, used especially to refer to something unpleasant.
例句
Most of the passes have been closed with the onset of winter...冬天來臨，大部分關口均已關閉。With the onset of war, oil prices climbed past $30 a barrel.戰爭一爆發，油價就攀過了每桶30美元。
英漢詞典釋義
n.
開始，開端，發生，肇始（尤指不快的事件）
at the onset of the war戰爭開始時The drug must be taken from the onset of the infection.這種藥必須在感染的最初期就開始服用。His first hit record marked the onset of an astonishing career.他的第一張風行一時的唱片標志著驚人事業的開始。
攻擊,襲擊
Our troops withstood the onset of the enemy.我們的部隊抵擋住了敵人的進攻。
英英詞典釋義
Noun
1. the beginning or early stages;
"the onset of pneumonia"
2. (military) an offensive against an enemy (using weapons);