noun, noun which may take the genitive case particle 'no', adverb
whole life; lifetime; all one's life
Refers to the entire span of a person's life, often used adverbially to mean 'for life' or 'throughout one's life'.
彼は一生を研究に捧げた。
He devoted his whole life to research.
一生忘れ
It's a memory I'll never forget for the rest of my life.
noun which may take the genitive case particle 'no'
of one's life; the ... of one's life
Used as a no-adjective to modify a noun, expressing that something is the most significant, greatest, or only instance in one's life. Common in phrases like 一生の思い出, 一生の不覚, 一生のお願い.
これは一生の宝物です。
This is a treasure of a lifetime.
一生のお願いだから、聞いてください。
It's the request of a lifetime, so please listen.