a young man in his prime: past boyhood, strong, of fighting and marrying age
This word names a man at the peak of his physical life. He is no longer a boy, but he has not yet aged: he is at the season of greatest strength, ready to fight and ready to marry. Older translations often rendered it "young man" or "chosen man," and both capture something true. The word comes from the same root as the verb meaning "to choose," so it can carry the flavor of a man in his choicest, most select years, the way we might speak of someone in the flower of life.
Because such men are a nation's strength, the Hebrew Bible reaches for this word at the two opposite poles of life. At the bright end, these are the young men who marry, who rejoice, whose vigor is their glory (the book of Proverbs says the glory of young men is their strength). The word is regularly paired with its feminine match, the young woman of marriageable age, as the picture of a people full of hope and future.
At the dark end, these same young men are the first to fall when a city is judged or destroyed. Prophet after prophet pictures the young men cut down by the sword, fainting from thirst, or carried off, precisely because killing them strikes a nation at its strongest point and steals its future. So a single plain word, "young man," can signal either a wedding or a massacre, depending on the scene around it.
Throughout, the meaning itself does not change. Whether the young man is rejoicing or dying, the word is pointing to the same figure: a male at the height of his powers.