REPROBATE (rĕp'rō-bāt, Gr. adokimos). The basic idea in reprobation is that of failing “to stand the test.” Adokimos, the negative of dokimos (“to approve”) connotes disapproval or rejection. When applied to humanity’s relation to God, it suggests moral corruption, unfitness, disqualification—all arising out of a lack of positive holiness. The KJV uses it of a reprobate [disapproved] mind (Rom.1.28) and of a sinful nature (2Cor.13.5, 2Cor.13.6, 2Cor.13.7). Its other NT uses (2Tim.3.8; Titus.1.16) bear the same disapproval quality. Human beings in sin are reprobate, disqualified, disapproved, and rejected because they cannot “stand the test” of holiness. Approval comes only in Jesus’ righteousness.
Paul says of the ungodly world, “Even as they refused (οὐκ ἐδοκίμασεν) to have God in their knowledge, God gave them up to a reprobate (ἀδόκιμον) mind” (Rom 1:28 ASV). RSV has “base mind” but this is inadequate. J. Murray (Comm. in loco) paraphrases it: “To a mind that is rejected because deemed worthless.” Paul uses athletic metaphors (1 Cor 9:27) in exhorting his readers to selfcontrol, and concludes by saying that he himself is not exempt from this need, for it is always possible that having preached to others, and not having heeded his own preaching, he might be “disqualified.” On the other hand, to those at Corinth who were suggesting that Paul was a counterfeit apostle who did not measure up to the standards of an apostle, he suggested that they examine themselves to see whether Christ is in them, otherwise they will fail to meet the test. Paul refers to some men who have a “counterfeit faith” (2 Tim 3:8) and to others who are “unfit for any good deed” (Titus 1:16). Hebrews 6:8 speaks of land which bears thorns and thistles which is “worthless.”
J. Denney in HDB IV (1902), 228; M. R. W. Farrer in NDB (1962), 1085.
International Standard Bible Encyclopedia (1915)
The close phonetic resemblance and etymological affinity of dokimos to the verb dokimazo, "to try," "test," has caused the notion of "being tested," "tried," and its opposite of "being found wanting in the test" to associate itself more or less distinctly with the adjectives dokimos and adokimos. Thus the more complex meaning results of that which is acknowledged or rejected, because it has approved or not approved itself in testing. This connotation is present in 2Co 13:5,6,7; 2Ti 3:8; Tit 1:16; Heb 6:8. In the first two of these passages the word is used of Christians who ostensibly were in the true faith, but either hypothetically or actually are represented as having failed to meet the test. "Reprobate unto every good work" (Tit 1:16) are they who by their life have disappointed the expectation of good works. The "reprobate (rejected) land" of Heb 6:8 is land that by bearing thorns and thistles has failed to meet the test of the husband man. It should be noticed, however, that adokimos, even in these cases, always retains the meaning of rejection because of failure in trial; compare in the last-named passage: "rejected and nigh unto cursing."
Cremer, Biblisch-theologisches Worterbuch der neutestamentlichen Gracitat(10), 356-57.