A legend in one's own lifetime
Literal meaning, that is, a living person of considerable fame.
The original use of this phrase was 'a legend in her lifetime', written of Florence Nightingale by Giles Lytton Strachey, in his well-known book Eminent Victorians, 1918:
The name of Florence Nightingale lives in the memory of the world by virtue of the lurid and heroic adventure of the Crimea. Had she died - as she nearly did - upon her return to England, her reputation would hardly have been different; her legend would have come down to us almost as we know it today - that gentle vision of female virtue which first took shape before the adoring eyes of the sick soldiers at Scutari.
She was a legend in her lifetime, and she knew it.
The 'own' is now almost always added to make 'a legend in his/her own lifetime', although I can't help thinking that Strachey's version was better.
The associated term 'living legend' derives of course from 'a legend in one's own lifetime'. This term sprang up in the USA in 1939 and immediately grabbed the imagination of writers there. In that year alone all of these people were described in print as living legends:
Jack Dempsey (boxer)
Cordell Hull (U.S. senator and Nobel Peace Prize Laureate (1945))
Diego Rivera (artist)
Fielding H. Yost (football coach)
The 'lost' Apaches of northern Mexico.
D. B. MacRae (journalist)
Strachey's phrase has spawned imitations. 'A legend in his own lunchtime' is often used humorously about chefs or notorious drinkers ("Lunchtime O'Booze" was used by Private Eye magazine as a generic term for a habitually drunken journalist). Less affectionately, there's also 'a legend in his own imagination', referring to those whose good opinions of themselves aren't shared by others.