Dictionary of Biblical Words:
(1 Cor. 9:27)
Translated also “reprobate” (2 Cor. 13:5-7), &c., and “rejected” (Heb. 6:8). It means one finally rejected by God. Paul’s use of it is not as having any such fear himself, but as a warning to others (especially at Corinth) to whom eloquence was all, and Christian morality very little. Paul frequently speaks in this way, as he explains in 1 Cor. 4:6.
Concise Bible Dictionary:
Paul kept his body under control, lest, though he had preached to others, he himself should be a castaway (1 Cor. 9:27). The same word is translated “reprobate” in Romans 1:28; 2 Corinthians 13:5-7; 2 Timothy 3:8 and Titus 1:16; and “rejected” in Hebrews 6:8. It is the negative form of “approved” (Rom. 16:10; etc).