complete, flawless; thoroughly prepared — having no defects or shortcomings, fully ready or secure (often used as 万全の〜).
出張のために万全の準備を整えた。
I made perfect preparations for the business trip.
万全とは言えないが、できる限りの対策は取った。
I can't say it's perfect, but I've taken every possible measure.
安全対策は万全にしておいてください。
Please make sure the safety measures are thorough.