1.
by some chance, if by any chance
Indicates an unlikely but possible scenario.
万一雨が降ったら中止です。
If by any chance it rains, it will be canceled.
万一遅れる場合は連絡してください。
Please contact us if by any chance you're going to be late.
万一火事が起きたら、すぐに逃げてください。
In the unlikely event of a fire, please evacuate immediately.
2.
one in ten thousand, remote possibility
As a noun, refers to a very unlikely chance or emergency.
万一に備えて保険に入った。
I got insurance in case of emergency.
万一のことがあったら、この番号に電話してください。
If anything happens, please call this number.
万一のこと = emergency, worst case
万一の場合に備えて、非常持ち出し袋を用意しておこう。
Let's prepare an emergency bag in case of the worst.
Literally means "one in ten thousand" (万 = 10,000, 一 = 1), suggesting very low probability.
COMMON PATTERNS:
- 万一〜たら/〜ば (if by any chance ~)
- 万一の場合 (in the unlikely event)
- 万一のこと (emergency, worst case scenario)
- 万一に備えて (in preparation for emergencies)
SIMILAR EXPRESSIONS:
- もし〜たら (if - general)
- もしも〜たら (if - emphatic)
- 万一〜たら (if by some unlikely chance)