early 13c., "an act or expression of disapproval, rebuke, etc., for something deemed wrong;" mid-14c., "responsibility for something that is wrong, culpability," from Old French blasme "blame, reproach; condemnation," a back-formation from blasmer "to rebuke" (see blame (v.)).
mid-13c., "having merit," from worth (n.) + -y (2). Old English had weorþful in this sense. Attested from late 14c. as a noun meaning "person of merit" (especially in Nine Worthies, famous men of history and legend: Joshua, David, Judas Maccabæus, Hector, Alexander, Julius Cæsar, Arthur, Charlemagne, Godfrey of Bouillon -- three Jews, three gentiles, three Christians). Related: Worthily; worthiness.
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Harper, D. (n.d.). Etymology of blameworthy. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/blameworthy