early 15c., "rejected as worthless," from Late Latin reprobatus, past participle of reprobare "disapprove, reject, condemn," from Latin re- "opposite of, reversal of previous condition" (see re-) + probare "prove to be worthy" (see probate (n.)). Earliest form of the word in English was a verb, meaning "to disapprove" (early 15c.).
1540s, "one rejected by God," from reprobate (adj.). Sense of "abandoned or unprincipled person" is from 1590s.