rep'-ro-bat: This word occurs in the English Bible in the following passages: Jer 6:30 (the Revised Version (British and American) "refuse"); Ro 1:28; 2Co 13:5-6,7; 2Ti 3:8; Tit 1:16. In all these cases the Greek has adokimos. The same Greek word, however, is found with other renderings in Isa 1:22 ("dross"); Pr 25:4 ("dross"); 1Co 9:27 ("castaway," the Revised Version (British and American) "rejected"). The primary meaning of adokimos is "not-received," "not-acknowledged." This is applied to precious metals or money, in the sense of "not-current," to which, however, the connotation "not-genuine" easily attaches itself. It is also applied to persons who do not or ought not to receive honor or recognition. This purely negative conception frequently passes over into the positive one of that which is or ought to be rejected, either by God or men. Of the above passages 1Co 9:27 uses the word in this meaning. Probably Ro 1:28, "God gave them up unto a reprobate mind" must be explained on the same principle: the nous of the idolatrous heathen is permitted by God to fall into such extreme forms of evil as to meet with the universal rejection and reprobation of men. Wettstein's interpretation, "an unfit mind," i.e. incapable of properly performing its function of moral discrimination, has no linguistic warrant, and obliterates the wordplay between "they refused to have God in their knowledge (ouk edokimasan)," and "God gave them up to a reprobate (= unacknowledged, adokimos) mind." Even Tit 1:16, "unto every good work reprobate," affords no instance of the meaning unfit, but belongs to the following rubric.
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.