fail-safe; foolproof; totally safe
Describes a system, method, or situation that is designed to be completely safe and free from risk of failure or harm. Often used in technical, engineering, or everyday contexts to emphasize absolute reliability.
This system is designed to be fail-safe.
Please save the data in a foolproof way.
安全 means 'safe' or 'safety' in general, while 絶対安全 emphasizes absolute, guaranteed safety with no possibility of failure.
万全 means 'perfect' or 'flawless' and is often used in phrases like 万全の準備 (perfect preparation). 絶対安全 focuses specifically on safety, not general perfection.
Compound of 絶対 (ぜったい, 'absolute') and 安全 (あんぜん, 'safety'). The combination straightforwardly yields the meaning 'absolute safety'.