early 14c., of persons, "diseased, wounded," from un- (1) "not" + sound (adj.). Similar formation in Middle Low German unsund, Middle Dutch ongesont, German ungesund. Meaning "morally corrupt" is recorded from c. 1300; that of "not mentally healthy" is from 1540s. Sense of "not based on reasoning or fact" is attested from 1590s. Related: Unsoundly; unsoundness.
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