just in case; as a precaution
Used when preparing for an unlikely but possible emergency or problem. Often appears with verbs like 用意する, 準備する, or 持っていく.
万一のために傘を持っていく。
I'll take an umbrella just in case.
万一のためにお金を少し多めに用意した。
I prepared a little extra money just in case.
Combination of 万一 (まんいち, 'by any chance') + の (possessive particle) + ため ('for the purpose of'). The phrase literally means 'for the sake of the one-in-ten-thousand chance'.