n.[theS][(+of)]

1. 开始
   He went to see a doctor at the onset of a fever.

   他开始发烧时便去看医生了。

2. 攻击，袭击
   Our troops withstood the onset of the enemy.

   我们的部队抵挡住了敌人的进攻。

n.

1. the beginning of something, especially something unpleasant.
2. archaic a military attack.