expression
knowing full well; being fully aware
Often used in the pattern 百も承知で or 百も承知の上で to indicate that someone does something despite knowing the risks or drawbacks. Conveys a nuance of 'I know perfectly well, but...'
危険は百も承知で、彼は救助に向かった。
Knowing full well the danger, he went to the rescue.
百も承知の上で、あえて反対意見を述べた。
I voiced my objection, fully aware of the consequences.
A similar emphatic pattern meaning 'no matter what' or 'in any case', but less common than 百も承知.
Literally 'even a hundred is known', where 百 (hundred) emphasizes completeness. The phrase likely developed from the idea of knowing something so thoroughly that even if you were told a hundred times, you would already know it.