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.