More than sufficient; plentiful to the point of being ample or exhaustive. Often used as '十二分に' to mean 'fully' or 'thoroughly'.
準備は十二分に整っているので、安心して始めてください。
Preparations are more than sufficient, so please start with confidence.
彼にはその仕事を任せる十二分な経験がある。
He has more than enough experience to be entrusted with that job.
説明は十二分に行ったと思いますが、質問があればどうぞ。
I believe I explained everything thoroughly, but if you have questions, please ask.