i-adjective
permanent; lifelong; never fading
Describes something that lasts a lifetime or never disappears, such as memories, scars, or bonds.
あの日の思い出は一生消えない。
The memories of that day will never fade.
この傷は一生消えないかもしれない。
This scar might be permanent.
消えない simply means 'does not disappear'; adding 一生 specifies the duration as 'for a lifetime'.
Compound of 一生 (いっしょう, 'one's whole life') and the negative form of 消える (きえる, 'to disappear').