The Scripture indicates that King David's middle son, Abonijah, was raised to do his own thing (1Kngs 1.6) without direction or correction from his father. This left Adonijah with a squishy middle, without the strength and depth of a godly man. What C.S. Lewis would call "a man without a chest." Abonijah's chestlessness plays out when David is near his final days. Abonijah takes the opportunity to crown himself king of Judah and Israel. With partying, celebration, and self-indulgence he proclaims his paper crown. It was all done without God, by a man without a chest. And it all failed miserably. When Solomon, the rightful heir calls Abonijah to the mat Abonijah quickly wilts and cries in fear because he (to his credit) realized he acted thoughtlessly and without God. In humilitaion he bows before Solomon and spares his own (miserable) life.
The Bible shows that men (and women) who serve God prosper. Those who 'man-up' succeed. In Biblical terms to "man up" is to be a man with a chest, a strong man who serves God.
Shortly before his death, King David gave Solomon this advise. "Be strong, show yourself a man (man-up) and observe what the Lord your God requires....so that you may prosper in all you do and wherever you go." (1 Kings 2.2-4)
The Bible is clear, Man up = serve God.